Places that water is present are generally layered. If one layer of protection fails there will be a second to prevent damage. This is true of properly constructed roofing and exterior components but not the other area in the home that sees a lot of water: the bathroom.
The seam between the bathtub and the flooring is especially vulnerable and is sealed with a flexible caulk. The subfloor is usually an engineered particle board and can be very sensitive to water exposure. The purpose of the caulk seam is to try to seal this joint as the two different materials move independently. The more water the seam sees the more likely it will open up, and particle board acts like a sponge that expands as soon as it is exposed to moisture.
I see grout used between the tub and the floor often, on home inspections, but it is not a very good solution. The tub is made of a different material and the grout will fracture and break.
Caulking is the best but it will need to be maintained. That means when it fails the caulk should be removed and a new, fresh seal should be applied. Seriously, when the caulking joint opens up, it is time to get in there with the razor blade and remove the old caulk. This should be done before damage to the flooring occurs.
Do not put this little maintenance item off unless you want some major changes to the flooring. If you want a comprehensive list of things in your home that you "ought to get around to," in the Salem, Oregon area, call me.