What's the plastic for?

My 2 youngest Pearls and I were out shopping today.  One of the things I needed were 2 specific colors of paint.  I quickly found one and Baby wanted to hold it.  I noticed it had the "keep fresh" or "for security" or whatever plastic on the lid and so felt she would be okay holding it.  I then continued to search and match the next color of paint that I needed.  Maybe about a minute passed and I looked at Baby.  She was smearing the paint all around her lips and then some.  I couldn't help but chuckle.  I knew she thought it was some kind of make up.  I caught her and her sister in some play makeup the day before.  Luckily while applying her blue lip stuff she kept her mouth closed.  And good thing for wipes.  They are always good to have on hand for who knows what.

So, really, what is the plastic seal on the lid for when it doesn't do any good?