Foaming is caused by pollutants in the water. Common culprits are make-up, body lotion and hair gels Anti-foam is an additive that reduces the foam formation, but unfortunately it can also make the water look dull.
If you are finding that you are getting a small amount of foaming after your tub has been used heavily then you need Antifoam to eliminate the problem. You might also find a scum absorber sponge useful. However, if you find that you are constantly needing to use anti-foam during normal use then you should think about changing your water and of ways to keep the contaminants out!