There's also a pragmatic reason: When sampling the PAL/NTSC signal cabling will have applied low-pass filtering, causing over- and under-shoot of the signal. By

There's also a pragmatic reason: When sampling the PAL/NTSC signal cabling will have applied low-pass filtering, causing over- and under-shoot of the signal. By setting the signal up to have the range 16/235 for an 8-bits A/D converter, this overshoot does not saturate the A/D converter. Lots of consumer electronics is designed with these kind of rules.