"Why have my weather radar on for taxi"
Probably so that it is stabilised, to prevent damage.
It will also give you a good view of weather on your intended SID.
The safety range of my radar is inside the radome, so you could stand in front of it in perfect safety (if very brave!).
Also, can't remember the formula, but it is based on the 1:60 rule, so, for example:
Tilt -2
Distance 30nm
Beam width 5 deg
Cloud is 2250' below (I hope thats right!!)