In short, a cookie is a file that I can use to identify you, or more precise, your computer or phone (I don't know if your cat walked across the keyboard and typed the URL by accident). I can store the cookie at mi casa or su casa, for legal mumbojumbo that does not matter, the point is I am collecting your user data.
Why do I need your data you ask. Well, first of, I assume the reason you are here is because you want to hire me to make great websites for you, right? I mean, the animations are nice, but that is not the real reason you are here. So simply said, I need this data to make better websites.
How is this data being used you ask. I can see the flow of your use, how long or short you look at a page, if you return often to one page or not. On an individual level this is not really interesting to me, but on a macro this data is very valuable.
I don't like to build websites to abandon them. More then half the time I'm designing a site from scratch, without user data (mostly because people don't need a site, when they already have a site). But when it comes to making a website better, it really helps to find snagpoints in the user flow. If I know where people leave a site, I can start finding out why they leave a site. And the same goes for successful pages.
Why am I willing to compromise privacy you ask. Well I don't think I'm compromising that much. First off, I can't see if you are that keyboard cat, or a real human. I don't track your privacy across multiple sites, so I don't really know what you are into, other then finding a webhead like me. And lastly, you connected to the server that I am renting, nobody forced you to come here (at least I hope not). It's a bit like walking in to a store and being angry that guy at the counter can see your face and says, hey you again, on a second visit.