Does Auxiliary fuse box need constant power?

[Greasey Harley]

I'm installing an aux. fuse box under the hood of one of my trucks. Happens to be a plow truck and has multiple extra circuits.
Plow controller, beacon light, aux headlights, aux backup light, electric fan.

It occurs to me that I should be able to run the whole setup off a continuous duty solenoid that energizes with ignition power.
Am I missing something?
Is there any reason to supply constant power to any-add on accessories?
If so, which ones and why?
Just trying to plan everything out while parts are still in the mail.

Thx
-Russ