FlatBread,
The greatest concern with engine anti-icing is many times on the ground, just before takeoff.
Takeoff thrust applied with little forward movement can result in rapid intake icing...clearly not good.
Engine anti-icing is used to prevent this problem.
Once airbourne, the +10C number (some designs use +5C) is used to be sure that anti-icing is switched on well ahead, to preclude intake icing.