Most automotive loads are only low side switched if they're using a solid state device as a low side driver, like most ECUs do. P-channel FETs, PNP transistors, BJTs, and IGBTs are all lower resistance (and usually cheaper) than their high side driver N-channel (or NPN, etc) counterparts. Less power dissipation, less voltage drop, most cost effective, etc.
Most relay circuits switch power for safety so there is no active power at a connector junction if the circuit is off. Smart switches (special MOSFETs designed to replace relays) are mostly all high side drivers, because most relays are used in that configuration.
All of the ECUs that we make (I work for an engine controls systems company) all have onboard P-channel low side drivers to switch loads, and all relays that supply power to the ECUs switch power, not ground. If an ECU switches a relay coil, it will switch the ground side, and share a connection with the power that the relay switches. This is usually the same with toggle switches that switch a relay coil (they often switch ground also). There are many exceptions to these rules, but most often relays switch power and control electronics switch ground.
Most of the choice for switching power or ground comes from economics, safety, convenience, or some combination of those.

Again, there is the same amount of current on either side of a load, so there is no power advantage to switching one side or the other. Some automotive relays have a metal case which is connected to the switch terminals, so when switching a load to ground the case is always connected to ground (not the switched set of contacts)..