There is some evidence that probiotics can improve bladder issues, but it's still an open question. The reason they help may not be so obvious. The bacteria in the probiotic don't somehow travel from your digestive system into your bladder.

Probiotics may help by doing things like improving the body's immune system so it regulates the bacteria in the bladder more efficiently or helps minimize unnecessary inflammation (our immune system seems to be powerfully related to our gut bacteria). Or since many pathogenic bacteria are introduced to the bladder from bathroom wiping technique, probiotics can help reduce the number of pathogenic bacteria in the colon, meaning there are fewer of them around to get inadvertently introduced into the bladder that way.