The more appropriate question is why do I by pass the water heater at the water heater rather than at the water termination center where I don't have to remove drawers and play contortionist to get at valves?
It's more force of habit than anything. I usually blow the lines out before using anti-freeze as well.
My routine is usually
By pass the water heater
Drain the water heater
Drain the fresh water tank
Open the low point drains (both sets)
Remove the water filter and reinstall the empty canister
Connect the compressor at 45 PSI
When water stops flowing from the drains close them off
Blow out the kitchen faucet
Blow out the Bathroom faucet
Blow out the toilet
Blow out the shower
Blow out the outdoor kitchen to include running the macerator pump
Blow out the outdoor shower
Blow out the low point drains a final time
Blow out the black tank flush at reduced pressure (25 psi)
Temps here will be in the low 30s later this week and although they are not supposed to drop below 30 I'm not willing to take the chance. After my Charleston trip I will blow everything out again, add anti freeze then take the coach to the dealer for its first and only warranty trip, chassis service, annual roof inspection, and state safety inspection. While it is at the dealer I will consider having them install the Direct Response system for the awning and roller shades on the driver's and passenger's side windows.
Sorry, long answer for a simple question.