Most dslr or mirroless cameras produce footage in a heavily compressed format. This is fine if you get everything right in camera but if you want to make adjustments in post then the footage can soon degrade. An external recorder allows you to record formats that are less compressed like ProRes which are easier to edit and have much more information allowing more scope for adjustment.
The other benefit with most external recorders is that they come with lots of tools like waveforms, false colour, focus peeking etc. and you get better sound recording.
If you are basically happy with the results straight from your camera and don't do a lot of grading and adjustment in post then you may not get a lot of benefit from an external recorder.