Well, often the reasoning isn't aerodynamic.
Without another design constraint there's no good reason to have an asymmetric intake to a symmetric engine face; you're just asking for distortion when you do that, as the air has to tarnsition to the symmetric section somehow.
Reasons for asymmetry can include:
nose intakes
flattened to provide view from cockpit
misshapen to provide somewhere to stcik a radar (e.g. Sabre)
underwing
ground clearnace (B737)
cheek, side, etc
anything on the side of the aircraft must be asymmteric, or at least split side to side, so not rotationally symmetric, which is what really matters. can be driven by nose radome needs, desire to shorten intakes (side to front face being shorter than nose to same front face)
I'm sure others will have other suggestions.
(And, of course, once you start getting away from pitot-type intakes all kinds of other considerations enter, such as ramp actuation)