There is definite value to them.
There are a few basic types: true primers, protective coatings, and those that are protective coatings, but can be painted over.
The true primers are meant to be painted over. You can weld through them with most processes on the materials they are intended for, but they don't provide a lot of protection themselves. Primers are meant to be painted over, and are not moisture tight.
The protective coatings provide better protection against oxidation than the primers do, but don't take cover coats as well, and the ones I have used are less mechanically sound. If in a protected environment, they hold up, but mechanical abuse that a decent paint can take will damage them. The one I use most is bloxide (same company that makes tempil stick temperature crayons). Several years of exposure with no other protection: no rust except where the coating has been worn off (clumsy forklift operator, for example). Doesn't contaminate welds, and is heat tolerant. Used on boiler parts, and holds up ok where not exposed to direct firing. looks like aluminum paint (basically is), but with a real fine pigment. Lays on in a very thin layer, so little binder (organics) to contaminate weld.
There are also types, including zinc bearing types, meant as permanent coatings with or without cover. I haven't used any of these much, but several types get good reports from people I work with. Don't know any brands offhand.
With ANY of these, the weld area will need to be recovered after welding, as they won't magically migrate over the weld, and will be burnt off adjacent to the weld.
I may not be good looking, but I make up for it with my dazzling lack of personality