one of the meanings is also - to appear -
Он явился вовремя. - He appeared on time. He was present on time.

являться - implies the idea of presence, quality of being.

If it's the meaning 'be' - it's more formal, right, like SS said.

Cf.

Это является серьезным аргументом. - This presents a serious argument.
Это серьезный аргумент. - This is is a serious argument.

The former sounds a little more formal.