It allows to use all of your favorite tools (eclipse etc.) without having to buy each one separately.
There are still some bugs in eclipse or maybe it's just me but sometimes when I open up my workspace there is an error message that says "The Workspace cannot be opened". But if you have already saved everything before then restarting Eclipse will work again so no big deal about this problem since its not too common with inno setup projects. Also creating new setups for software easily as well as using them from previous versions can also be done quickly through Inno Setup which makes things easier for our clients who don't know how to install programs themselves.