What if you don't think that you can come online for a couple of days?
Also, some people like to see the position with fresh eyes every day.
Why Have Days to Make One Move?
Some people like a longer time to think. Not me, but some people are like that. Also, if you don't like it you can play live chess!

Well, if you correspondence at less than 12 hours, it gets really difficult with sleep.
Besides, when it comes to correspondence chess, which is designed for you to take your time, you are somewhat obligated to use the tools that have been given such as the analysis board, which is incredibly helpful.
I found that when I started using the analysis board, and I was taking my time on moves, I was making fewer mistakes, and these minor mistakes can sometimes cost you the game.
If you want to play faster moves, play live games, I find that with my busy schedule, live games are difficult to put in, and I am struggling to keep up with 44 different correspondence games going on, and when I have that many, I find it best to let the time go while I stick with one game and analyze to find my best move.
So, in conclusion, Correspondence Chess is designed for the most perfect play any person can make, per their own level. Live chess, including OTB, is better for playing your best under a time constraint. Even if you are not playing with a clock, you would not usually spend more than a day to finish a game, and if you are taking days to make moves, its closer to correspondence chess anyways.
Hope this helps :)

Less than 12 hours would not work because people need time to sleep and go to school/work.
I DONT

Why not be content with a variety of choise? People have different needs and that variety helps each in their own right. Just because you can't find correspondence chess meaningful, doesn't mean somebody else wouldn't. You don't have to play 900 games either, but if it suits someone else, why not?

Why should you have a day or more to make a move?
One move per day can happen in correspondence just from different schedules. Even if two people are in the same time zone, player A might always do his chess for an hour after dinner from 7 p.m to 8 p.m. His opponent player B might always do her chess from 7:30 to 8 a.m. If they're never online at the same time and each only logs in to play chess once a day, there won't be more than 1 move per day, even if they only spend 5 seconds thinking about the move. Different time zones and work schedules have the same result. That's one of the nice features about correspondence chess-- you're not forced into a particular daily schedule in order to be able to play in any tournament or against a specific opponent.
Why even longer periods? Someone who needs a game schedule with a long period doesn't necessarily take that long for each move, but they might need the long period on occasion. I might make 2 or 3 moves each day during the week, but go camping every weekend and never log in on Saturdays and Sundays, so I would need all my games to have at least a 3-day turn-period. You have to have a turn-schedule that will cover your longest regularly expected gap between logins, even if it doesn't happen very often. There aren't enough vacation days available to cover regular expected gaps, and some tournaments don't allow vacation days at all.
--Cystem

I came to this same conclusion, being relatively new to this site. I see some people select a 3-5 day move rate, and wait until an hour or two before the game times out, or even miss it and lose that way. I honestly can't figure out why someone can't make at least a move every 2 days or better yet, every day. I am going to start limiting my game requests to one move a day.....

Why should you have a day or more to make a move?
One move per day can happen in correspondence just from different schedules. Even if two people are in the same time zone, player A might always do his chess for an hour after dinner from 7 p.m to 8 p.m. His opponent player B might always do her chess from 7:30 to 8 a.m. If they're never online at the same time and each only logs in to play chess once a day, there won't be more than 1 move per day, even if they only spend 5 seconds thinking about the move. Different time zones and work schedules have the same result. That's one of the nice features about correspondence chess-- you're not forced into a particular daily schedule in order to be able to play in any tournament or against a specific opponent.
Why even longer periods? Someone who needs a game schedule with a long period doesn't necessarily take that long for each move, but they might need the long period on occasion. I might make 2 or 3 moves each day during the week, but go camping every weekend and never log in on Saturdays and Sundays, so I would need all my games to have at least a 3-day turn-period. You have to have a turn-schedule that will cover your longest regularly expected gap between logins, even if it doesn't happen very often. There aren't enough vacation days available to cover regular expected gaps, and some tournaments don't allow vacation days at all.
--Cystem
What he said. I wonder if anybody, other than Prawny, is here on the weekend.
I came to this same conclusion, being relatively new to this site. I see some people select a 3-5 day move rate, and wait until an hour or two before the game times out, or even miss it and lose that way. I honestly can't figure out why someone can't make at least a move every 2 days or better yet, every day. I am going to start limiting my game requests to one move a day.....
And that's the way it's supposed to work. No one ever has to play games longer than they are comfortable with. If all you want to play is 1 day/move games. That is perfectly fine. And those of us who prefer slower games can play as we wish.
Why should you have a day or more to make a move? I can understand a couple of hours, but a day?! If you are one move away from checkmate, you could probably forget it the next day. I think that if someone creates a seek and then gets off for a week that it should be deleted a day after.