I have 123 games in my Steam library, less than 1/3rd of them are installed due to lack of space on my 1TB platter drive. 90% of the time they can sit on any format of drive and it won't matter, but the other 10% of the time they need performance and they need it right away. They're too large to be continually deleting and re-downloading, so the ideal is to have them on an SSD so they have the performance when they need it.
That's just games not including video rendering applications, audio manipulation, etc. It took me less than 5 days to fill 1TB when I first got this machine - on a monthly basis I'll churn between 750GB and 1.6TB of data between 2x1TB drives according to my ISP and that's not including local uses of which there are numerous that require both performance and lots of capacity.