Ad credits are ad packs that you purchase to advertise your website, product or service to our members. They are NOT used for you to see more ads and you do not make money off purchasing ad credits.
Advertising will not give you any benefits to your ClixSense account. You will just pay other members of ClixSense to watch your advertisement.
But if you advertise something, the profit from advertising might come from your advertisement itself.
For example, if you advertise a product and some users who view your advertisement(s) like it, they may buy it, so you will get potential customers.
You may advertise your referral link of another PTC site that you are member of. And if other users like your advertised site, they might sign up and end up being your direct referral to that other site.