1) Because you can.
2) Because 30 isn't the limit to what all can see.
3) Because sometimes you don't see it, you feel it.
4) Because high-end GPUs will mean you don't have to upgrade as soon again as otherwise.
5) Because great GPUs have great features (GTX 780 records gameplay).
6) Because some GPUs come with free games (Such as some Radeon cards).
7) Because you can't do it on consoles.
8) Because you want to brag.
9) Because there's worse things you can waste money on.
10) Because you might be using multiple-monitors; yes for work too.
11) Because powerful GPUs can speed up other programs you might use often (Video/photo-editing).
12) Because you've got a 120Hz monitor (that already adds fluidity) and you want a GPU to match.
13) Because high-end GPUs are a thing of beauty, for many.
Note, no one in their right mind, on these forums, would buy a GTX Titan.
Some might get a GTX 780 though.