Why must we post to this forum for get a free space in trap17 ???

Well back in the day or many many years ago, posting was used as a way to get your site advertise in such a way that every other post would contain some sort of ad, but to achieve that ad quota you need people to post and thus a post limit was set up. Which was used to pay for the free hosting; however, since a few millions sites did this OpaQue decided to build a credit system in order to get rid of the need to place an ad on every post and eventually phased out the whole advertisement gig altogether.

But for the long run posting is free way of advertisement especially on the search engines and so the more posts you have on your site the more links are shown on the search engine. But here's the trick with free hosting your usually limited in what you could and it supposed to gear you towards paid hosting on that websites paid service. Of course that doesn't happen here but eventually your going to start having space issue and bandwidth problems such running out of them 2 days after you get a new set of BW.

To simplify my answer; basically posting is a free advertisement and your helping in building that free advertisement by posting; and so with all the posting you get you should be able to get something in return and thus your free hosting.