Simply coz it caters for people with enough hash rate to get a valid number there also.
I guess if you enter a small hash rate I could ban you from seeing that number if you thought that was appropriate ... ... ... ... ...
However, no one has ever donated a single satoshi to that web site in the years it's been there, so I only change it when I feel like it, not when people complain about it trying to cover their mistakes

I guess it would help if you actually bothered to look what's there also ...
Scroll down and it allows you to estimate the diff change and thus get a BTC estimate based on your choice of diff changing.

Yep, caters for all sorts of people to get estimates