Some games really require it. If you don't there's no way to keep up in what they expect you to. It's like there's a huuuuuuuge gap in difficulty if you don't. Suddenly everything will near one shot you or completely overkill you. Don't forget the daily (monthly) bonuses and time limited events to get exclusive characters which only unlock certain items or abilities it expects later on for some runs to help get through another trial, etc. It's all kinda unfair to expect everyone to pay up just to keep up in the difficulty and...it really just turns off the game's quality.