I know, you can search without it. You can click them however and it takes you straight to the page! (yes, useless) Still, it is easier to search for tutorials with #PHP than with PHP since other people talk about PHP and I am not interested in that.
Those are subjects. You can use them to search for other posts related to that subject.
Personally, I really like twitter. It is a great means of advertising. Also, because it is really short, I don't have to read a lot of text and I can just skip over the subjects I am not interested in (I am following a lot of companies who have tutorials for web development).
I work at a supermarket and I saw that 2 items were gone (which needed refilling) they were taken from the same row and there was a row next to it the same way but that one was still full. I can still not believe the thing that went on in my head: "Damn griefers".