If not properly primed, porous surfaces can absorb a lot of paint, which can result in an uneven finish and force you to apply more coats of paint than you normally would have to.
Brand new drywall and wood surfaces are extremely porous, so if you are painting a wall that has never been painted or painting over areas of wood paneling, use a primer first.
If the wall you are painting has noticeable stains, use primer first to help the paint easily cover them and to prevent them from showing through the paint again in the future.
Use primer first when you’re painting over a very dark color with a much lighter color, which will allow the new paint to cancel out the old paint and keep it from showing through.