What is personification and why would an author or poet use it?

Personification is when something inanimate/not living (or sometimes an animal) is presented as human and living, for example, in Act I, Scene II of “Romeo and Juliet”,

“When well-appareled April on the heel
Of limping winter treads.”

Shakespeare has here personified the month of April and the season of Winter - April cannot wear clothes, as "well-appareled" suggests, and it cannot walk, let alone tread on the heel of Winter, which itself cannot limp and does not have a heel! This technique makes the description far more vivid - rather than just saying that Spring was coming and overtaking Winter, we are given a beautiful image that really draws attention to this change in seasons. It is far more effective and memorable than a simple description.