it's not about space, actually ssd sucks when it comes to space, well it does not suck, but it will suck your cash if you try to by anything close to 1TB.
the main reason is speed, i have an old ssd, and i boot at around 13sec. And what i have is far from the fastest. Loading games (and apps) improves by a lot.
most of the time, ssd for OS (and a few games/apps), then the rest goes to the mechanical hdd