What can a scientist learn (only) from patent literature/database rather than from research literature?
This may vary by field, but in my opinion most patents are not very useful for a researcher. The patents that I have read are all written by attorneys in attorney-speak.  Trying to figure out what is the real technical content behind the attorney-speak is difficult if not impossible.  

To give an example: I used to work at a large company.  My company wanted to patent some of the work that I had done. My boss had me send my material to the attorneys, and they drafted the formal patent application based on my work.  When I received the patent application back, I could not even understand what the patent was talking about, and it was something that I had literally invented.  So good luck to anybody else trying to understand what it means. 

Occasionally I have come across some good technical content in patents, but it's usually in the situation where the researchers have filed a patent first and then published a journal article immediately afterwards.  They did the patent just to protect their intellectual property.  In that case the patent does have some good info, but the corresponding journal article is even better.

Again, your mileage may vary.  In some fields I guess this might be different.