What causes the line?

[#1]

Somehow I creat these lines under my layer in the timeline

What causes this and how do I make it go away?

[#2]

I figured out how to get rid of them but I would still like to know their function.

[#3]

The function is easy visual identification of layers when you have tens of them and long timelines.

[#4]

Yes, the tool is very important and distinctive.