The creators sell adventisement space and want the advertisers to know that their channel is a good investment, so the more they can prove to the advertisers that their sponsor segments arent skipped, the more they can charge for it.

It's the default web interface - it doesn't show on all videos, but in my experience it's on nearly everything.

Youtube does provide info on which portions of videos are the most watched - while most advertisers aren't the kind of people that do due diligence, quite a few of the big management groups have started introducing contracts that base payout for sponsor reads off of actual watch count. AFAIK it hasn't made too much of a difference yet (though channels with high skip-counts are less likely to be given the decent sponsor deals) but if youtube makes the analytics easier to access it probably will have a pretty big impact.