Convenience really.

When in the UK, I live in York, which is an expensive 400 mile round trip to the Phils embassy in London,
as I am not willing to put my passport in the post to them.

Hence, I (when not using my Balikbayan one year stay privilege) either ask for the 29 day extension when arriving at NAIA, Manila, or just go along to my nearest Immigration office at a later date.