I mean sure, its usable, but its not good. You can notice the differences in buffering / scrubbing speed well into the 100+ mbps range.
Plus being able to download and upload files quickly. Particularly from something like a home NAS, is important. 15 mbps is like using a shitty USB 2 stick for everything!
The point here is that only devices like a TV, mobile, tablet or laptop should be on WiFi and it's pretty hard to notice the difference between say 50Mbps and 500Mbps on any of those except maybe if you are moving files around on your laptop.
Your smartphone is not talking to your NAS over Ethernet.
Traffic is bursty. Higher bandwidth connections make the whole internet more efficient - if you can race to idle then servers have fewer concurrent connections to keep track of, routers can more quickly clean up their NAT tables etc etc