When designing a database, you want to make it possible to look up information. Think about finding a post in this forum. I don't just want to all the posts that you made. I also want to look up threads, do searches, etc. If I had all your posts in one CLOB, they would be extremely difficult to find.
A much more logical approach is to have a posts table so we can lookup each thing individually. This forum uses the database structure provided by jforum.net. (with a few things added on.) I recommend looking at their site if you are curious about the exact schema.