So, there are lots of reasons why SaaS rocks as a way of delivering functionality to customers. One of them is the ability to make good use of GPL'ed software.
At a regular closed-source software company, you can't include any software in your products that you get from the web which carries the GPL (Gnu Public License), because this license requires all "derived works" to also use the GPL and be open source. This is why the GPL is often called the "Gnu public virus", the license contaminates the software it touches.
What's worse, there's a more limited LGPL, which is supposed to allow linking to other software without having the GPL provisions apply to the whole linked "work" of software. Though this would seem to fix the problem, many software companies don't allow use of LGPL software either. (Reasons here seem to be more paranoia about patent restrictions, than real, but lawyers are lawyers.)
Here's the trick: The GPL is troublesome only for companies that distribute software. Here's where the SaaS idea wins. We don't distribute our software. We're a service provider. We have no problem at all basing our service on software built from GPL'ed pieces. We're in perfect compliance with the license.
The GPL recently was updated from v2 to v3, and there was worry that v3 would close what is called the "service loophole". This didn't happen. GPL v3 still allows the service loophole. They decided against closing it.
Net result: there's a huge and growing body of open source software, and the largest part of it carries the GPL license. As a SaaS provider we can exploit this base of code to do what we want without fear.
This is a big advantage over "ordinary" software.