Because you're going from 0.15-3 to 0.21-1, where major library changes have occurred, efibootmgr heavily depends on libefivar, and efibootmgr/efivar have a history of obscure bugs from corner cases. You're free to avoid these extra steps if you like, but given the tight relationship between efibootmgr and efivar, I wouldn't, particularly for such a critical pair of packages.
Steve:
Because you're going from 0.15-3 to 0.21-1, where major library changes have occurred, efibootmgr heavily depends on libefivar, and efibootmgr/efivar have a history of obscure bugs from corner cases. You're free to avoid these extra steps if you like, but given the tight relationship between efibootmgr and efivar, I wouldn't, particularly for such a critical pair of packages.
efivar/git$ git log --oneline Debian/ 0.15-3. .Debian/ 0.20-1 | wc -l
121