So, is this really necessary?
necessary would be a bit of a strong term, perhaps "useful tool" would be more appropriate, but it's providing the opportunity to express agreement and/or thanks without clogging the thread with quote+text. if a post earns 10 thanks/likes the message is clear and the author of that post knows that his contribution was welcome/well taken.
there are many enough forums with a thanks button, it also can and does add to measure the quality of content that a user is generally providing, visible in the ratio between posts and thanks/likes. this again can help moderate a lot.
one perfect example of this would be "XDA-Developers.com forum but there are many more, and in this context it has nothing to do with "friend" status like in facebook. it's a moderation tool after all, see attached screenshot and the link to see how it works. http://forum.xda-developers.com/showpost.php?p=65135292
the example only should illustrate the way how it works and can look, not related to this forum in any way !