I’ve always been interested in the watchers (Grigori) for their darker celestial nature. I do reckon they are “angels of the black cosmos”. But beings like Semyaza, Azael, and Darkat are rarely described in personal experience.
I was wondering what kind of earthly gain these entities can bring to someone who works with them. (If anyone has personal experiences to share)
Earthly gain? I’m not sure. Much wisdom can be gathered from them that can be translated to material gain.
I have some experience with them, not enough to answer your question. However, I’ll give you two book recommendations that you can use to answer your question.
The Pathworkings of the Watchers Churaki Ashan
The Book of the Watchers Ars Aurora
Isn’t “The Pathworkings of the Watchers” like 42 pages long? 
I’ll probably check out “The Book of the Watchers” though. Thanks!
Serious pathworking can occur from a single sentence. Haha
It can occur with one word, the spirit’s name spoken out loud. But I was looking for more of a grimoire kind of deal.
Oh, okay.