5 minute blitz games are perfect for familiarizing yourself with openings.
Slow games have cons as well:
-cannot fit as many into one session (i.e if your practicing an opening)
-may lose focus because game is going on too long
-most people online don't want to play super long games (e.g. g/45 d10) and even this is very fast compared to OTB
-Chess is about having fun, If you are not having fun, you shouldn't be playing, I often find blitz games very fun and long games kind of tedious (opponent can stall for a while)
Pros of blitz:
-fun
-practice openings
-learn how to deal with time trouble
-develop intuition
-not very serious or stressful
-can fit a lot into one session
there are some bad things about blitz as well but it can be treated as a tool for learning and for improving ur chess.
Slow chess is a test of long-term calculation and strategy. Blitz is a test of intuition and spur of the moment improvising.
That's already been said or implied in previous posts.