Question: In Cap. VII of LLPSI, Ørberg introduces *Advenit* with the following sentence

> 
> Ecce Iulius ad villam advenit.
> 
> 
> 

It's curious to me that the verb includes the preposition; why not just use venit alone rather than advenit? Is it that *advenit* is meant to imply coming to a specific place, whereas *venit* is more general?

Answer: 
In Latin, it's fairly common to stick prepositions onto the fronts of verbs to create new shades of meaning. Sometimes the new meaning is the same as the verb plus the preposition (*advenīre* is basically the same as *venīre ad*), sometimes it's just plain intensive (*dēplorāre* is just *plorāre* but stronger), and sometimes it creates a new meaning different from the sum of its parts (*interficere* means "kill" or "execute", not at all the same as *inter* or *facere*).

In this case, it's the first. *Advenīre* is basically like *venīre ad*—"to arrive at". *Advenīre ad* puts even more emphasis on the arrival.
It is perhaps not easy to work out the relevant meaning differences if you only compare *advenire* and *venire*. In order to better understand the role of the prefix, it can be useful for you to consider the "full" picture/**system**, which involves comparing *advenire* with other prefixed verbs like *pervenire*, *obvenire*, *evenire*, etc. You'll then discover that each prefix adds a particular directional/path nuance. 

As for the repetition of *ad* as a preverb and as a preposition in your example, it should be noted that this phenomenon is typical of so-called "weak satellite-framed languages". Latin and Slavic languages are classified into this typological (sub)set. Please take a look at [this link](https://latin.stackexchange.com/questions/11083/are-latin-verbs-of-motion-satellite-framed-or-verb-framed) for an answer to a related question on motion verbs.