+1 that the electric pump is the backup during takeoff and landing in case the mechanical pump fails. When you're at cruise altitude you'll have time to troubleshoot, but near the ground you can't count on having, and likely won't have, time to deal with sorting out what happened and turning on the electric pump. At least, I'm not gonna count on that.
Another utility of the electric pump is that if the mechanical pump gets overheated for whatever reason it can boil the fuel and cavitate during it's suction draw stroke. The electric pump will pressurise the input to the mechanical pump and make this much less likely, or mitigate it if it happens. I have to use the electric pump during hot starts at high DA for this reason.
The mechanical pump and the electric pump are designed to be reasonably fail-safe with each other. In other words, the diaphragm-type mechanical fuel pump in many Mooneys will allow fuel to flow through it if it fails if there is pressure from the electric pump. Likewise if the electric pump fails (or isn't turned on), the mechanical pump can still draw fuel through it. These common failure modes are all tested and have to be demonstrated during certification, so they're all tested, safe states.
Also, the fuel servo is essentially an output fuel pressure regulator, and it is largely insensitive to input fuel pressure as long it is high enough to allow regulation of the required output pressure. Increasing the input pressure doesn't really bother it, which is why you can turn the electric pump on or off and not see a change in engine output parameters. It's extremely difficult to overpressure the input of the fuel servo, so that's nothing to worry about.