ATX Motherboard

Not Dell, HP, etc. Prebuilts with proprietary components, but the ones that use off the shelf components and sell in major retail stores. They seem to always use a full size ATX case with a micro ATX motherboard. Is there any point to this apart from saving costs with the motherboard but making...

I just bought a Rosewell 2U server case and it states that it accepts Micro ATX motherboards. Has anyone attempted to put an ATX motherboard into one? I am going to attempt to move my computer components into a server. Thank you