After drywall has been laid and it has been plastered. A primer sealer is highly recommended. Essentially this is the absolute last stage in home building. Left without primer plaster will eventually corrode and chip or break away and drywall has a porous paper layer. It’s typical to see drywall without primer in garage and basement areas. It can work occasionally but in any areas with traffic and humidity it could play risk.
When priming drywall you also want to prime for the your coat of paint. If you chose not to prime the new drywall then the paint will not adhere properly to the surface, it may stay on the wall but when it “challenged” it will more than likely scratch and/or show it’s lack of adhesion.
When you decide to repaint your condo, house, office, etc you may need primer again. One reason is when patching nail holes or other minor holes you will need to prime those little patches of plaster or filling agent. If the patches are not primed then they will flash; meaning that there is a visibly a slight difference between patch and the surface area around the patch. The difference generally shows itself as a difference in sheen, the patch will appear “flatter” as the surface is rougher.
The only issue can be predicting which primer to use. You should generally use a lower-end value brand for this. If you choose a high-end primer you may be sealing the surface and doing the exact opposite by producing a glossier sheen difference. This is due to the strength of the primer. If it is a very potent mixture it will seal the area of the wall creating a less porous area and thus a glossier surface for the finish paint.
Another consideration when choosing a primer to repaint is your final paint colour choice. The challenge here is going from very contrasting paint colours. For instance if your wall colour is yellow and you want to go purple then you may want to use a colour tinted primer. A tinted primed is typically a lower-grade primer with colour added. One reason for this is that primer is formulated to hide the surface it is going over, another reason is primer can be cheaper than your finish paint and the initial primer coat is a step towards the finish paint colour. So instead of three coats of paint that is generally more expensive than primer, you can do at least one with your tinted primer and get by with less paint usage overall. The question comes down to whether or not the pigments in the paint is of a high opacity.
In older homes however you may need to prime all the surface before painting, because you can never be to sure about the substrate and what type of paint or substance is on any of the numerous layer of paint. For instance, When applying latex over oil/alkyd based paint you’ll always want to apply a primer suited for this purpose.
Water damage is an area where primer is made to block the water. When dealing with water damage you want to assess the cause before moving forward with the paint job. Painting can only do so much and some times the job will need to be reffered to a general contractor who can fix the source of the water damage. If the problem is fixed then a cautionary water block primer wouldn’t hurt.
Some areas where primer is used include areas with marked with markers or crayon. Some of these tools can leave a nasty mark on the wall that is extremely visible through multiple layers of paint, sometimes the only way to fix these areas is literally cut them out of the wall and re-plaster. Usually this can be rectified with the right pre-paint primer application if cleaning them proves futile.