I am after some information on approach plates, in particulalr why is an approach generally given in degrees on a precision approach and in a percent on a non precision approach.
I think the short and useless answer is "because Annex 4 requires it".
Annex 4 says:
11.10.8.5 Final approach descent gradient and, in parenthesis, descent angle to the nearest one tenth of a degree shall be shown for non-precision procedures with a final approach fix.
11.10.8.6 On charts depicting ILS/MLS and LNAV/VNAV approach procedures, the height of the ILS/MLS and LNAV/VNAV reference datum to the nearest half metre or foot and the glide path/elevation/vertical path angle shall be shown.
11.10.8.7 Final approach descent angle shall be shown to the nearest one tenth of a degree for instrument procedures with vertical guidance.
Why? I don't know.