What is the purpose of the sessionid on internal links?

[BirdOPrey5]

I noticed in one template I had had made some internal links but didn't add the sessionid to the link. It's been this way for years, works fine, and no one has ever complained... So what is the point of adding the sessionid variable to internal links?

[Zachery]

To users who browse without cookies, and to guests who browse without cookies.

[BirdOPrey5]

thanks. :)