If you've developed for Microsoft's SharePoint before (I'm talking about 2007 here, but this applies to WSS2 and 2010 as well) , then you'll know that you can reach the limits of it's functionality very quickly. This is a big problem if you're making a zero-code solution, i.e. you have no access to Visual Studio and can't create web parts. This is more common than you'd think, especially in large organisations that use SharePoint extensively. For this, the only choice is to use SharePoint Designer 2007 (SPD), but it's not pleasant because, frankly, SPD sucks. I've not found a program that crashes as much as SPD, or that performs so poorly when presented with the most basic tasks. If you make a page that is too complex, has too many web parts, large data sources or lots of conditionals, connections and filters, it can take anywhere up to 20 minutes to perform a single action.
There's another benefit for working this way around: your code will work on standards-compliant browsers, and any that come along in the future. This is always a good thing as you don't know when the organisation will roll out IE8/9 to its users, nor can you always guarantee that a user will be using a IE. It's important that sites are ready for these changes and best-practice development is maintained.