A primer is the foundation coat you put down before your colour. It does three jobs: it seals porous surfaces so the topcoat doesn't sink in unevenly, it promotes adhesion so the paint actually grips, and it blocks stains, tannins and old colours bleeding through. Skip it where it's needed and you'll get patchy sheen, peeling, or water marks and knots ghosting through weeks later.
Here's when you genuinely need one:
Bare or new substrates. New plaster, bare wood, MDF, new plasterboard — all thirsty and need sealing first. New plaster wants a mist coat (watered-down emulsion) or a dedicated plaster primer once it's fully cured. Bare or knotty wood needs a wood primer, and for resinous knots reach for Zinsser BIN (shellac-based) to stop them bleeding.
Stains and problem surfaces. Water marks, nicotine, damp patches, marker — these laugh at ordinary paint and need a stain-blocker. Zinsser Cover Stain or BIN are the trade go-tos.
Changing surface type or going glossy-to-matt. Painting over old gloss, tiles, melamine or kitchen units? Degloss and prime so the new coat keys in. For cabinets specifically, Bedec MSP or a Rust-Oleum cabinet kit are sound.
Big colour changes. Going from a deep colour like Hague Blue to a pale like Pointing or Wevet, a tinted grey primer cuts the number of topcoats dramatically.
When can you skip it? Sound, previously painted walls in good nick, same-ish colour, same finish — crack on with two topcoats, no primer needed. Most of the supported brands (Dulux, Crown, Little Greene, Farrow & Ball) sell matched primers and undercoats, and many modern emulsions are self-priming on sound surfaces.
Practical advice: don't overthink it on a straightforward repaint, but never gamble on bare wood, fresh plaster or stains. The primer is cheap insurance against a job that fails six months down the line.