And getting new versions of your software out by trying to get X distributors to include your newest version in their repositories (which are completely outside of your control and each with it's own set of rules) is less complicated than just putting new stuff up on your own website? I don't think so. There is one advantage for users certainly - it's easier to find versions of software which are officially sanctioned by their distributor, although it might be outdated by months. Which does often not matter much for server-software, but for desktop software it's something which users don't really accept.
I think we can discuss if this central-control-by-distros model of software distribution has more advantages or disadvantages, but it's not all shiny throughout. As example when stuff go really wrong with that model - maybe you heard of the troubles with Debian, J.Schilling and the cdrtools. I didn't until I noticed I could no longer burn CD's(!) with k3b and had to spend a few hours on figuring out how to fix this (you have to get the Schilling versions of the tools - the official Debian-version simply does not work in some cases and that is known now for a long long time). So there is a well-working combination of k3b+cdrtools which fails to pass the distributor rules (which is certainly fine) and now the authors can't really get the working combination (of 2 open source packages!) in an easy way out to the users.
I think we can discuss if this central-control-by-distros model of software distribution has more advantages or disadvantages, but it's not all shiny throughout. As example when stuff go really wrong with that model - maybe you heard of the troubles with Debian, J.Schilling and the cdrtools. I didn't until I noticed I could no longer burn CD's(!) with k3b and had to spend a few hours on figuring out how to fix this (you have to get the Schilling versions of the tools - the official Debian-version simply does not work in some cases and that is known now for a long long time). So there is a well-working combination of k3b+cdrtools which fails to pass the distributor rules (which is certainly fine) and now the authors can't really get the working combination (of 2 open source packages!) in an easy way out to the users.