"The boy walks near the girl."
Translation:El niño camina cerca de la niña.
Think of it using "close" instead of near. In English we require "to the" or "by the" after it. "He walks close the girl" doesn't sound right, because it's missing the preposition. This is a close parallel to what you'd be saying in the Spanish sentence if you omitted "a" or "de"
a cerca de is about time or distance acerca relates to the meaning