There seems to be many Japanese verbs that are mysteriously compounded with 出る, like 出来る and 出会う. What is the actual semantic role of the 出? I'm especially curious about the difference between 会う and 出会う.
Since there is already a question for 出来る, I'll add my two cents for 出会う.
I think 会う and 出会う have clear distinct uses. 会う means "to meet", as in
I went to see my mother.
This type of meeting someone is usually planned. (Of course あう can mean more than meeting someone, but these uses are not comparable to 出会う any longer.)
出会う means "to come across, to meet by accident, to encounter", e.g.
Sorrow is at parting if at meeting there be laughter.
The 出 just means what it always means and the structure of 出会う is just ます-stem + verb, like in 見合わせる, 作り直す, etc. I don't think it's too far a stretch to imagine that "to go out and meet" came to mean "to encounter" or "to meet by accident".