If I was using an engine, I wouldn't need 2 days per move. ;-)
chess.com is very good about finding cheaters. Not to say there aren't any - but the chances of you playing any at the 1500 level are pretty slim. The cheaters are going to be well over 2,000, and they have to be very careful now, or they will be found and banned.
To answer your question as to why people need the time. I'm playing about 15 games at the moment. That's not very many. Some people are playing hundreds of games. They need two days per move just to make all of their moves on time. I like to have that time when I'm in a difficult position. I don't just give up and make a crummy move - I leave the board alone for a few hours or maybe sleep on it. Then I come back with a fresh perspective and hopefully a move I haven't thought about will occur to me. I try hard not to take all my time on any move, but there have been occasions where I just felt like I was missing something, and I did take all the time to find something.
I hope that answers your question. The cheaters don't need 2 days!
How do people play this chess where u need 2 days per move? Arent people just using engines?
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