There’s at least some evidence that probiotics do nothing to change gut bacteria in people who are already healthy. The studies that tested it seem to show that the ‘new’ bacteria introduced by probiotics just pass through you and make no difference to your overall GI health.
That said, if you don’t have a healthy GI system it could be that probiotics can help normalize and re-populate your gut with “good” bacteria. Anecdotally, I’ve had good luck using kefir it when I’m getting over a bad GI infection that has left things generally “off”. It seems to reset my system. My pet theory is that a bad GI infection leaves my gut bacteria a mess and this helps repopulate things. Again, totally anecdotal though!