Do you actually need primer?

A sound painted wall is one situation.

For a clean, dry, sound wall with compatible existing paint, your selected topcoat may allow direct application. Check its product data. Repair damage first, and spot-prime repairs when the coating system calls for it.

Changing from dark to light can still take additional coats. Primer and topcoat serve different roles, so do not count a primer coat as one of your planned finish coats.

Bare or unevenly porous surfaces are another.

New drywall, exposed joint compound, and bare wood need a system suited to their different absorption and surface characteristics. Ask for the primer specified for that substrate and your chosen finish paint.

A common mistake is coating a patch and the surrounding wall identically without considering absorption. The patch may remain visible as a change in sheen even after the color looks covered.

Stains and slick finishes need a specific answer.

Identify and correct the source of a stain first. A water mark from an active leak is a repair issue before it is a decorating issue. Some stains require a dedicated stain-blocking primer.

A glossy coating, laminate cabinet, metal surface, or unknown old finish may require a bonding primer and different preparation. Bring a photo and describe the surface to the paint store. Test adhesion as directed before coating the whole project.

Ask a better question at the counter.

Instead of “Do I need primer?”, say: “This is previously painted drywall with new patches, and I want this finish paint. What preparation and primer does that system require?” You will get a more useful answer.