Certain accountants say that you must keep the BP in order to prove that you took the flight (I've got plenty of BPs from flights I ended up not taking but no self respecting bean counter ever lets a fact stand in the way of paperwork.) They usually cite "IRS regulations require..." in much the same way that FAs quote "FAA regulations require..." when they're about to say something particularly nonsensical.

As a result some companies require that you submit them with your expense report.

Bureaucrats will eventually bring down this civilization under the weight of their paperwork...