"I worked for an assistant researcher" means the assistant researcher was my boss, I reported directly to him/her.
"I worked as an assistant researcher" means that I was the assistant researcher.
You can only work for a person (that person is your boss), you can work for a certain company (work at the company), or you can work for a certain wage (I work for $20 an hour, etc).
When you say "work for the benefit of," I wouldn't actually use "for" in this case. Instead of saying "I work for my children," when you mean that you work in order to provide for your children (for their benefit), you would say something different - "I work to support my children / I work so that I can provide for my children."
(not a teacher, just a language lover)