Caulking the connection between your shower walls and tub prevents leaks and reduces mildew buildup. Removing old caulk and recaulking joints between tiles and your tub is an annual bathroom maintenance job.
If you have tile shower walls, there is a very good possibility that a leak could develop between the tile and the tub. As the house moves (a natural process that occurs in every home), a hairline crack can occur that allows water to get into the joint. Once water gets in, there is no telling how much damage can be done.
There's another good reason to recaulk. If you've tried to remove the black mildew stains from caulk, you know the blemishes are often unbudging. That's because the stains are often behind the caulk — between the caulk and the wall. The answer, of course, is to remove the caulk, kill the mildew, and then replace the caulk.
After the caulk has softened (it may take a few hours), remove it with a plastic putty knife. Clean the joint with paint thinner and wipe the area dry with a clean rag.
Then clean the joint with this famous mildew cleaner:
Immediately after eradicating the mildew, use a hair dryer or a hot air gun to thoroughly dry out the area. The joint can now be re-caulked.