In Lingua Latina per se illustrata, chapter 7, there are several examples of phrases where a compound verb, using a preposition as a prefix, is used in conjunction with a lone preposition:
Quid inest in saccīs?
Iūlius ad vīllam advenit.
Iūlia ē cubiculō exit
What is the purpose of this repetition? For example, in place of the second sentence, why not either of these options:
Iūlius vīllam advenit.
Iūlius ad vīllam venit.
Does a lack of repetition here make the sentence ungrammatical? Or is the purpose emphasis, or something else?