Because it's a Boeing procedure, in my Part B and in the FCTM.
Less flippantly, Boeing are targeting a level acceleration, so a mode that aims at level or near level flight works well. Modes that work are ALT HOLD or VS. I think at JMC it was ALT HOLD, ALT HOLD to fly a level accel. Now it's VS 0-200 which works well. Once at Ref80 FLCH will give a speed mode which will climb at Ref80.
FLCH for the accel will drive towards the selected speed but with no direct control over the gradient.
EAM has answered the approach flap thing.
Hope this helps.