It's based on the persistent & false conviction that this will help to prevent bites...the kind of bites that might occur if/when you reach into a snake's cage & he is
expecting food & bites you by mistake when you only want to handle him. The belief is that the snake will learn it's only feeding time when in the side cage.
But it doesn't work like that. First, snakes get stressed when handled...sometimes that puts them totally OFF eating [in a side cage]. Then you still have to put them
back in their own cage afterwards: snakes stay in feed mode for hours or even a day, so you're more likely to get bit when you do this. So much for the "advantage".
Additionally, if you handle them awkwardly when returning them to their home, they can also regurgitate what you just fed them.
The best thing is to remember how our snakes sense the world (including us): great sense of smell & touch, vision not so good (best for motion detection, but not for
accurate identification). And snakes like BPs also have heat sensing pits to detect warmth of prey. The idea is not to confuse them & instead give clear signals so they
know what's coming, & there's several ways to do that: "hook training" (a gentle touch using a snake hook or similar harmless item to reach out & touch your snake)
let's them know it's handling time, not food time. Or scent- I use this method with my snakes (non-BPs), I just blow air across my hand in their direction so they can
realize I'm not prey...it's immediately clear to them & they back up (somewhat disappointed, lol). I have a few larger & more excitable snakes that sometimes I like to
use one more signal with for clarity, & that is a simple spritz of water (in their face, when they persist). That seems rude, I know, but it's no worse than getting rained
on...it's a very effective way to "change their channel".
So considering the downsides of feeding in another cage, & the ease of communicating your intentions, you're right, there's no reason not to feed a snake in their own
cage where they feel the most secure & relaxed.