Theoretically buying views, likes and subscribers for a new channel could increase the watch time on videos, my thinking being that when people see a video with a lot of views and a good like/dislike ratio they subconsciously give that video higher value and are therefore more likely to give the video a chance. I do it myself, I'm sure I do. If I come across a video with a low number of views on a channel with a low number of subs, and the video starts slowly, I am likely to bail pretty quickly. However, if that video has a bunch of views and the channel has a bunch of subs, subconsciously my mind associates those things with good content. Something like "well, if this many people watched it, and they have this many subs, there must be something good about it. I'll give it a couple more minutes".
That is the only understandable reason for anyone to purchase views/subs. Basically, for appearances, which could help watch time metrics. The downside, of course, is that fake views tend to hurt watchtime metrics. If someone told me they bought 500 views on each of their videos as a head start, I'd be curious to see how well that strategy served them. Many of those who sell views/subs on various black hat type forums maintain that they are 'high retention' but I have yet to see results that point to that not being a bunch of nonsense.
Adwords is a much better option, when done right.