There were some older model scanners from the 90s or maybe very early 00s that required you to enter a P25 band plan before they could track the voice traffic. I suppose it's possible that there are a handful of P25 systems out there that don't broadcast a band plan, so you'd have to figure it out and enter it manually. I believe someone recently discovered just such a system, although it would be extremely rare.
Band plans in scanners are mostly a holdover from the old Motorola Type II days, where you'd have to tell the scanner how to calculate the traffic channels, as that info was not transmitted over the control channel. That's where all of the 800 MHz rebanding came into play. Oh and a real subscriber radio needs the band plan to be entered into the codeplug, aka the ASTRO 25 Channel ID table in Motorola parlance.
