One of the great things about OBS is that it gives you the options you need for both streaming, and offline recording.
I personally use the recording feature quite a lot, and with that I edit videos a lot. While editing, having my mic audio separate from my desktop audio is incredibly helpful.
In OBS, these options are usually found under - Settings / Output (Output Mode : Advanced) / Recording
From there you can establish which tracks you'd like to record via the check boxes , and you can assign each device to whichever track you'd like from the mixer (Advanced Audio Settings below your preview)
- E.g. Desktop Audio : Track 1 | Mic/Aux : Track 2 -
From there, your recordings can simply be dragged onto a timeline within an editing software (I personally use Sony Vegas), and the two tracks will automatically split from each other. Alternatively, I believe it can be done manually. In that case you'd have the separate audio files. Though with either method you'd still be given the freedom to edit the levels of one track, without effecting the other.
When it comes to recording from Streamlabs OBS, the files don't do the same thing, as the option doesn't seem to be available at all. This is despite you being able to assign your desktop to one track, and mic to another in the same mixer under the preview.
If I haven't just made some sort of mistake and skipped over the option entirely, I'd like to suggest that it's added, as I'd like to switch to Streamlabs OBS for it's other features that aren't included in OBS, and I feel more users would enjoy that feature as well.