The particle に is a "grammar" word, so it is not a good idea thinking of it as "meaning" any list of English words. For particular verbs, you have to learn "case-by-case" what particles it uses to hook to different types of object. (This is very like the way you just have to learn which preposition to use in French, or which case in languages like German or Russian.)
Anyway, in this particular case consider two sentences:
ともだちに 会います。 (takes に)
I meet a friend. (direct object)
ともだちを 待っています。 (takes を)
I am waiting for a friend. (preposition 'for')
Now imagine a Japanese friend asks you: "Why do you say 'waiting for a friend', and not 'waiting a friend'." How do you answer? Probably just "Because that's what we say"...? In which case you have the answer to your own question. And if you find that unsatisfactory, imagine the Japanese friend saying "Well, I find that unsatisfactory..." You can sure see where this goes.
Another verb you have to learn a similar pattern is のる. Riding a bicycle (direct object) is じてんしゃに のる (particle に）. Again you just have to learn it, but there are not so many of these that it is a big problem.