Whatever your tuning project is, if you think about adding a diffuser, then you are not wrong for a few reasons.
And if you are going down that road, you might as well grab one from RKP, who do a decent job of it.
If your ride is either an E92 or E93 BMW M3, then you just have to go with their brand new diffuser. And you want to do this for several reasons. First off, the looks alone should warrant one, as it can nicely complete the visual aspect of a tune.
But there are also some performance improvements to be had here. Diffuers usually go on race cars, and their roles is to streamline the airflow under the car and provide even more downforce. While equipping it on an M3 might not make that big of a difference, some people just have to add every little bit. Also, installing new exhaust systems sometimes mandates that you get a diffuser.