primers help to supply any finished painting exercise a much more uniform appearance both in terms of colour and sheen, which of course makes it much more attractive. This is especially true when the surface being painted is porous or uneven in porosity. welcome to
I see a distinct difference between primers and sealers. To me a sealer stiffens and seals the wood fibers. For a sealer I use a 50:50 mix of urethane varnish and thinner. The wood sands much easier after sealing.
To me the primer fills small pores, scratches and such, and protects or moderates between the surface and whatever paint you use. It does also make a uniform flat finish which makes it easier to see imperfections. I now use a primer I learned about on this group many years ago. It used to be called Krylon Sandable Primer. It is now just called Primer. It is very full bodied (lots of filler) in it so it fills small cracks and sandpaper scratches such very nicely and dries quickly. I have used it under hobby enamels, acrylics and even lacquers and it seems to stand up to anything. However, it WILL craze acrylics if used OVER them, and sometimes slightly affects some enamels, so complete your priming as much as possible before using color paints.