Hi,
That's a little bit simplistic but essentially correct. The differences in your example above are not that you would need to develop a complete Expenses Management System, you could simply amend the existing forms, lists and workflows to suit your envisaged Expenses System if you wanted. But if you do want a totally customised Expenses System, MOSS provides you with a huge set of pre-developed tools such as (to name but a few); All your authentication done, access to the user profiles information from an LDAP source, a simple way to integrate workflow, all your data storages needs, most of your presentation needs, a huge set of ready to us components that can be used as a starting point, the list goes on...
MOSS does have a lot of functionality OOTB but in most cases where I've been involved, everyone wants to tailor it at least a little. This is where the benefits comes in. A couple of hours work can produce a great system, whereas starting something like an expenses tracking system from scratch in ASP.NET with the complexity of workflows, approvals, email enablement, alerting, security, chinese walls, etc could take months. Have a look at the Forms server aspect, utilising IE7 and MOSS to provide a very complex business forms routing and workflow based set of functionality to meet your needs.
Then of course, there's always to option buy a simple plug in and have it working in minutes (if it suits your needs). The main point is that this system is very flexible and should be seen as a platform rather than a black box.
I work with some of the largest banking companies in the UK and everyone of them is using it and deploying it to one degree or another. The reason? It's a great time and cost saver versus traditional development of web delivered services, pure and simple.
Cheers
Paul Leigh