Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: What is its function
Document: i was really excited about this item but i dont think it is going to function at all. it is way to big for my tub and about 30% of it doesnt have a place to stick on to.boo.
Super product!! Works perfectly! Have one I have been using for two years and no deterioration in function!! If you like a FULL tub-GET THIS!
This product is great. For function I give it 5 stars. It is very hard to clean though and I am sick of trying to wash the mildew out all the time. Buy this, but don't store in your shower. It gets NASTY and is impossible to clean!
This cover won't stick to my bathtub; therefore it won't perform its function at all. It sticks for mere seconds and then is off. I have tried every trick in the book to get it to stick, all to no avail. Don't waste your money. Just fill the tub with less water and you'll be better off than using this product.
This really stops the overflow function of the tub knob -- even my non-standard one (it isn't a nice flat surface, but has an indentation where the knob is. Just wet the little suction cups and put into place. With mine, I have to do a little manipulation (and not try to stick the cups all the way around) it stops the overflow function and I can have a nice, deep, spa bath!
works perfectly! allows me to take a deeper, more relaxing bath. very pleased. I got this as well as the tub stopper and I am finally able to take a bath without worrying about water drainage.
works just like it says it does... helps keep the bath full, no complaints, and it gets used very often!
works great.. just like it is suppose to.. i was able to fill my tub all the way up and there was no leaking at all.. NONE.. that is awesome.. i thought for sure there would be a slow leak and the water would make its way down.. but nothing.. i wish i had bought this years ago.. works awesome.. for and awesome price
works great. my tub has a little bit of texture and it still stuck. also due to the shape of my tub i can only sticks 2/3 of the suction cups to the tub but i still had no trouble.
works great does what its supposed to do.. suctions cups keep it firmly in place while you fill the tup
Answer: Dear Mary,  This covers the bathtub overflow drain so we can fill our tubs deeper.  My home has overflow drains installed at a ridiculously shallow level.  Can't have a relaxing soak in 4 inches of water. This is an effective, low cost solution.  Works fine.