Lids for glasses

I've recently noticed that there are a few whiskey glasses which are sold with a lid (mainly Glencairn it seems), or that there are lids available for sale. As I am quite the beginner when it comes to whiskey, I was hoping someone could explain the reason for having lids on your glasses?

Some obvious reasons come to mind, such as keeping insects or similar out, but something tells me there's more to it.

Thanks in advance!

EDIT: Added a link to a picture, to clarify what I mean.