as webmasters, we were assuming that a +1 on its own was a good thing for our sites. but maybe another user now has to confirm that the +1 was useful before google starts giving our sites a boost?
after all, the whole +1 thing was just another way for google to measure the quality of a site. so this new 'thank' thing must have a similar purpose.
i reckon that is what its for. google must be inundated with spam +1s. so they have introduced this new thank thing to weed out which ones really count.