Selenium ide is good for testing features that have mostly visual elements (links, text and etc.). But often web apps have features that don't represent itself as visual elements, like sending emails, queueing jobs, communicating with 3rd party services and etc. You could, for example, test if an 'Email has been sent' message is present after submitting a form. But it doesn't really tell you if an email is actually sent and therefore you aren't really testing the whole feature here.
