They can be. Some motherboards come with one built in. But in most cases it refers to its own PCIe card, such as one of the many models from LSI Megaraid.
The advantage of this is that it can have a small capacitor bank (or a proper battery) to provide emergency power so that if something stupid happens such as motherboard failure, the raid controller will use this power to cleanly write to the disks.
EDIT: I just remembered one such stupid situation at work where a motherboard died and then the entire system blacked out, including power to the drives. I spoke with my vendor since data loss and corruption carries a hefty price tag in my field. They told me not to worry - The data could sit in the buffer for ages, as the capacitor bank was there to handle things like this. Turned out that upon restoring power, once the array was online again, the write buffer will be written to disk. No CPU or motherboard required - the controller took care of it. This was especially handy since it took a little longer to find a replacement board.
Here in Norway the scale goes from “Ferrjævelig” to “utepils”. Numeric scales is an EU thing we didn’t sign up for.