Question

Why do some people paint outdoor gates during rainy season

Answer 1

doors are usually made of iron
they can be in touch with moist air and water during the rainy season.
paint prevents the air and water to connect with iron
so some people paint their outdoor gates

Answer 2

some people paint outdoor gates during rainy season because
there are chances that the  gate may rust as the rusting happens when metals come in contact with with the moisture painting them from outside cuts off the contact of moisture and metal and hence prevent it from rusting