Questions: use of P3 Brake Controller

I, too, am planning an exchange of brake controllers from my current Reese to a Prodigy P3 proportional controller. I have a couple of questions:

1. The P3 can toggle back and forth between for use with electric braking or for use with “electric over hydraulic braking” systems. I assume that when it is used with a TM trailer only the electric braking feature would ever be in use. Correct?

2. On eTrailer's site the listing for the P3 (for use with a 2010 Ford-150) there is also a sidebar ad for an “In-Line Circuit Breaker” – 20 or 30 amp. Checking around for possible reasons why anyone would want an easy way to “take out” the P3, I found only a couple of possibilities:

a. Use the Circuit Breaker to disallow the Controller to drain the battery when vehicle is not in use, and/or

b. Use the Circuit Breaker to disallow the Controller to govern trailer brakes when backing into a site that has a slope at some point.

Am I correct in any of the above? Are there other reasons to use or not to use a breaker or to use the "electric over hydraulic" option?

Thanks.