Is footage recorded through external recorder really better? Most DSLRs offer 8-bit recording, 4:2:0 with 100mb/s 50/mb/s 500mb/s in their own containers.

Why would shooting with an external recorder (let's say ProRes HQ or 422), make the picture look better or easier to grade?
Amount of mb/s difference is huge between internal and external.

Still did not get an external recorder, but before I do, would like to get a clear understanding of this. Of course, bigger LCD and lots of SSD space is a huge plus.
Unless the DSLR/Mirrorless camera you are using outputs 10-bit 4:2:2, using an external recorder to up-convert the footage to ProRes HQ will not result in a better image quality than what the camera is able to produce/output.

If anything it will allow you to edit and re-compress the footage multiple times without losing the original quality.

Using an external recorder may result in a better picture quality compared to the internal encoding, because there is more bitrate available per frame, more robust codecs, and also being able to remove the 5, 15, or 30 minutes recording limitation the camera may have.

A bigger plus using an external recorder (or display) is the ability to nail the focus, assess proper exposure, and having multiple eyes monitoring the recording.