Why to buy Github watchers?
Lets put it this way, you have high number of forks on your most starred project but there are zero watchers. What impression does it give to people checking statics on your repository?
Github Watchers for you
Definitely it shows that people who have starred or forked your code, they are not interested in being updated about any progress done on it OR probably your project inst as useful as they initially thought it was. To be able to gain visitors immediate trust, having reasonable number of watchers is essential.
What is more important, Github followers or watchers?
None is less important than other. It all depends on one thing, and that is, what is your goal? Followers are fellow users who are following your over all work. However, watchers are people who are specifically watching progress on your one code/repository.
So if you want to give better impression of your over all profile, you should aim to get more followers. If you would like to boost visibility on one special project/repository, you should aim on getting more watchers for it. In simple words, increasing number of watchers shows how popular your specific project is becoming.