Blowing bubbles helps children develop the muscles in their mouth and tongue, so that their pronunciation and speech will be more clear. Which sounds more fun to you - blowing bubbles every afternoon or attending speech therapy? Bubbles will not cure all speech problems, of course, but they can help either prevent or improve mild speech concerns.
Bubbles teach cause and effect in a way that young children can understand, without the adult needing to intervene. For example, if your child dumps out the bubbles, they'll be all gone! If you do not refill, your child will quickly learn to change their behavior. If you do refill, that's "rescuing" and you're on your way to becoming a "helicopter parent".