Why might this relay be in use?

[DeadheadNM]

I’m slowly unwinding all the electrical changes made to my 99 Suburban that was formerly operated by a municipal fire department.

This relay appears spliced into an engine harness bundle at one end and the passenger side battery at the other end (red wire with an in-line fuse). The truck dies if this wire is removed at the battery. At the harness end it looks like a red harness wire was cut and spliced to the blue and yellow wires from the relay.

Not sure why this modification was done other than a possible anti-theft option. Any other thoughts? I don’t have the schematic for this truck like I do my 72s and want the wiring back to original.