Question

Why is precipitation heaviest near the equator

Answer 1

The air is very warm and warm air results in a lot of evaporation. The air sucks up moisture from the surrounding ground. Warm air rises. This warm moist air rises and gets cooler, condenses and clouds form then the the heavy rains start. Once the rain is over and the skies clear, the air warms up, sucks the moisture back up, rises, cools etc. etc. This cycle continues year round. Tropical rainforest areas — rains heavy every day for a short while. 
It is like here (N. America) in the summer. On really hot days you can get severe thunderstorms - same reason.

Answer 2

Cause the sun cause the ocean warmer to evaporate