The iFrame's module is an IE only option to bring in static or external content to a site. It lets you link to most types of files and is best used when working with smaller page types to make it totally integrated.
An example here is an email opt in registration form that has post back functions within the site. But other uses include working on quickly grabbing content from non DNN sites and pulling into DNN.
Using iFrames is independent of DotNetNuke sites so you can't utilise membership role behaviour or module funcationality like integrated DotNetNuke modules have, but do have a degree of usefulness in many instances.
Sometimes, the iFrame module opens up a world of flexibility to get that bit of code, that particular file, anything that you simply can't put into a DNN site easily, you can usually implement into your DotNetNuke site by utilisng the iFrame Module.