TLDR stands for too long; didn't read. It's commonly used online to give a brief summary of something that is otherwise very long. People use it because they don't have the time or patience to read through everything, or because they want to get to the point quickly.
There are a few reasons why people might put TLDR at the beginning of their posts. First, as mentioned, it saves readers time by letting them know that there is a lot of information and they may not want to read it all. Second, it can be used as a way to tease readers and get them interested in what you have to say. By saying that your post is too long for most people to bother reading, you're essentially dared them to prove that they're different and will take the time to read it all.
That said, there are also some drawbacks to using TLDR. For one thing, it can come across as arrogant or presumptuous, since you're essentially telling people that your writing is not worth their time. Additionally, if people do take you up on your offer and read through everything, only to find that it wasn't worth their time after all, they may be disappointed or even angry.
Ultimately, whether or not you choose to use TLDR is up to you; there's no right or wrong answer. Just be aware of the potential consequences before you decide one way or the other.