"this bug affects me" vs. "I can confirm this bug"

In Launchpad's bugtracking system, I get the opportunity to click a button named "this bug affects me" ("affect-button"). I assume the more people click this button, the more (community) developers get interested in it.

However, in several bug reports comments appear containing the only line "I can confirm this bug.". My two questions are:

- Are those commenting people just ignoring the "affect"-button or is there a reason why they write those lines?
- What does the "affect-button" actually do?

Your answer will change my behaviour in confirming bugs.

Thank you for your help. I hope, some more people will learn from your answer.