Confused about "patch by channel" and "patch by dimmer"

I'm trying to learning the patching. I have some dimmers and some DMX lights. So far I managed to control he dimmers using "Patch by dimmer". And for "Patch by channel", I do 1-to-1 patching (ie. Channel A 1 to DMX 1, and so on), but I do it because I don't actually understand why/what's that for.

So could anybody clarify what's the difference / usefulness of "Patch by channel"? I'm confused about the difference between these 2.