"She comes for a visit."
Translation:Hon kommer på besök.
The practical use of prepositions varies greatly across languages, and Swedish (and English!) is no exception. För does mean "for", but that doesn't mean you can translate it like that regardless of context or what's idiomatic to say.
To say "hon kommber för ett besök" is not wrong though, but it shifts the meaning of the sentence slightly to sound more like a formal and planned visit, perhaps like what a doctor would say about a coming patient.