Personally, I filter cards by format legality more often than by individual sets or anything else, most frequently Modern and Standard. When building a deck in those formats, it's of course crucial to filter by the allowed cards.
Now, the filter doesn't quite work as expected, really - it shows you cards from within the editions in the format (so right now, it has Theros block, M15, then Khans block), but that's not technically correct - for example, if I wanted to include a Battlefield Forge, the reprint in M15 is just as legal in Standard as the Apocalypse one from way back. To me, it would be tremendously helpful to display those cards as well, not only for perfection's sake, but because it would be comfortable to directly be able to see which edition I could grab a card from if I don't have a playset from the most recent edition, for example.
Another application is to see the various promo cards such as FNM versions showing up while having set filters applied.
I imagine a filter could work by a rule such as 'get all cards from selected editions, then get all cards with the same name from all other editions'. As the extra editions are hidden by default, that wouldn't be in the way for anyone either.