Yes that was me. I stumbled across the question, read it, read the answers, and thought ‘wow, yes, in my opinion this is what an answer on Stack Exchange look like; extensive, readable, I like it.’ Well, offering bounties is a preferred Stack Exchange way of saying thank you, if you are not the question owner and cannot accept. (Or if you are the question owner but want to reward two people’s answers.) See the help centre.
As per the underlying system, a bounty can be offered on every question older than two days that is not closed. And by anyone who has achieved at least 75 reputation.
Bountied questions are kept in the featured tab for a week to hopefully draw even more attention to them and (hopefully) give the good questions/answers even more upvotes. At least for the subsequent bounty I offered on a different question after I awarded Terry his bounty that worked; at least five upvotes were dropped in the bounty period if I’m not mistaken. Featuring is ended if the bounty is awarded.
Of course there is a second reason for adding a bounty other than rewarding answers: attracting them. It doesn’t matter what the bounty is for, attention is always good. It will say in the bounty notification why it is there.
And please do not forget to add both questions and answers to the bounty dance!