I had a positive UTI culture but no symptoms? They prescribed me antibiotics anyway??

The doctor is telling me I have a UTI although I have no symptoms and I read if this happens that they won't give you antibiotics unless you're pregnant or about to have surgery because it's not a big deal however I was prescribed some.... I don't want to take them if I don't have to because apparently they can make my birth control less effective. Like I said I have 0 symptoms and I think possibly the infection could be somewhere else or could not be there at all. Don't most UTIs go away without treatment anyway?