See How to mark a topic as [Solved] for some reasons to close a topic.
In addition to this: With topics not being closed, they will accumulate more and more postings. Who will read 800 postings (with 5 not marked solutions in it)? Correct, nobody, and there you have postings 801. And 802....
With topics being closed, topics stay slim and readable, do not accumulate endless postings, and the solution is shown below the first posting for easy finding.