In NXT it makes sense, if they've had a good run and they're a big name down there, giving another wrestler a rub on the way out is the perfect way to go. The hype of them being called up will negate any feeling of them being weak following the loss.
basically its a tradition to help the next generation of talent as they leave because guys wouldve done the same for them when they were starting out, guys have to lose to make someone a winner so its only fair for the winner to lose to create a new winner. Its the more constructive option for the future.
Because you want your company to seem like it has the best employed by them. If you book a guy to dominate your roster and then he just leaves, you have a roster full of people who just got schooled by a guy who doesn't work there anymore. You don't have the best. The best went elsewhere.
On the flipside, if your guy beats him and he leaves, he looks like a chump who couldn't hang with the new best wrestler in he world and tapped out. It's best for your product (most of the time).
They did that last year before the draft, it's unknown if they will again, it worked fine for them though. They put someone over and when they switch brands they're around a set of people they haven't worked with in a year, they can build them from there because whoever they lost to is not around as a constant reminder of that loss.