Depends on your use, I need about 128GB for a pretty small app collection but with a load of Spotify & other music downloads. If you watched videos a lot on your phone too(I know many who do with these larger ones) then I can see how that could grow substantially further, same with some modern games which can be in the 3GB+ range.
Most importantly though, these larger capacities generally also come with higher speeds. If you've ever seen a phone model tested across various capacities, the larger ones are measurably faster in many general tasks(Opening apps and the like), and that gulf grows as they fill up(Which is why many low end 8GB and 16GB Android phones turn into slugs after a while, especially if they're already RAM starved and relying on the flash for virtual memory). With Samsungs new V5 NAND's they're getting 1Tb (128GB ) per chip, so 1TB would require 8 chips(Not a massive number by any means, especially if you're trying to keep a multi-channel controller fed), which also explains why the minimum is what it is.