You can use web services for many different reasons, but the most common is probably when you need to query data based on a given parameter. Infopath has a fairly good means of sending emails as it is, especially if you get the iLoveSharepoint addon at codeplex (http://www.codeplex.ccom/) for your sharepoint server.
As a general rule, you want to use as little coding as possible because it can get REAL ugly when you try to upgrade or install an addon. If you're doing this at the professional level, your product typically won't get support from Microsoft PTR's/Engineers with custom code.
There's a pretty good tutorial here for setting up web services, I think it's just on the main page part way down. There's one heck of a learning curve before you really get a feel for InfoPath, but once you do, you'll realize it's way more capable than you thought.
Good luck!