In English, prepositions can be added to verb phrases as complement or adjuncts. Some examples:
(1) I will put it down.
Here, down is a complement : put absolutely needs something there, you can't say
*I will put it.
(2) I will put it out.
Here, out must have been a complement originally, but something happened: this combination has obtained a fixed, specific meaning that must be listed in the dictionary separately: to extinguish. So we can say that put and out now combine to form a single verb with its own meaning.
(3) I will write it down.
Here, down is not a complement but an adjunct: I might instead say
I will write it.
What is more, it doesn't really add any meaning. It basically just acts as an intensifier. Apparently, using to write doesn't quite feel comfortable without an added preposition, without really requiring it - but the preposition hardly adds any meaning. Similar examples:
(4) I will write it out.
(5) I will print it out.
So we see that in English, it is really easy for prepositions to get stuck to verbs into a fixed combinations, without changing the way the verb functions in the sentence; this may produce a specialized meaning, but it may even happen when it doesn't really affect the meaning at all.
The same thing used to happen in Latin. A lot. For instance, it had a verb monere meaning to advise, and after due time, people would use admonere instead. In your case, the ad does change the meaning: tribuere means different things than attribuere, but the difference isn't very large. (You can look them up in Wiktionary.)