Primer is often treated as a single category that makes makeup last longer. It is several different products doing unrelated things to the surface underneath.
The surface is the actual problem
Skin is not a flat canvas. It has texture, pores, variable oiliness across regions, and areas that are drier or flakier than others.
Foundation applied directly onto that surface follows it. Product collects in pores and fine lines, and dry patches grip pigment more than smooth areas do.
A primer intervenes between the two, changing what the foundation is being applied to rather than changing the foundation itself.
Silicone primers work by filling
The most common type uses silicone polymers that spread into surface irregularities and level them, creating a smoother plane for the next layer.
This is why they feel slippery and why they visibly reduce the appearance of pores before any makeup is applied at all.
The trade-off is that a silicone layer can repel water-based foundation, causing it to bead or separate. Matching the base chemistry of the two products matters.
Oil-control primers work by absorbing
A different type contains powders and absorbent materials that soak up sebum as it is produced, keeping it from reaching the makeup film.
These extend wear on oily skin substantially, since the usual failure mode is foundation breaking apart as oil rises through it.
On dry skin the same ingredients pull moisture from the surface and make texture and flaking more obvious rather than less.
Hydrating primers work by adding
The third approach adds humectants and emollients so that dry areas stop absorbing foundation unevenly and the film sits consistently across the face.
This directly opposes what an oil-control primer does, which is why using one across the whole face when only part of it is oily produces poor results.
Zone application, matching primer type to region, addresses this and is the reason many people end up owning more than one.
Why colour-correcting primers are separate
Tinted primers work optically rather than physically, using a complementary colour to neutralise redness or sallowness before coverage is applied.
Because the correction sits under foundation, less coverage is needed above it, which keeps the finished layer thinner and more natural looking.
They do nothing for texture or oil, so they are frequently layered with a functional primer rather than used as a replacement for one.
Their effect is also limited by what goes over them, since a high-coverage foundation obscures the correction entirely and makes the extra layer redundant.