There are generally four (04) kinds of paint finishes.
Matt Finish- Little to no shine at all. This finish is great for covering imperfections on the surface but is not the most durable option.
Eggshell Finish- This low-luster finish provides a small amount of shine. Eggshell finishes are great in areas that do not get very dirty, (i.e. low traffic areas such as an entryway or living room.)
Semi-Gloss- This finish has more shine and reflective qualities than other finishes. It’s great for rooms that get a lot of traffic and steam such as bathrooms as it’s mildew-resistant.
High-Gloss- This finish is the shiniest, most durable, and easiest to clean. This type of paint is perfect for cabinetry, doors, and certain trims.
The simple answer is that primer is not always necessary. When you're repainting a wall just to change the color, you can usually omit the primer.
But primer necessary for new surface. If you skip priming for new surface, there have risk of peeling paint, especially in humid conditions.
However, in many cases primer will actually save you money because it reduces the number of more expensive topcoats you will need to get beautiful, lasting results.
Also you'll experience uneven coverage without a primer coat, especially along drywall seams. This means you'll need to apply more paint to finally get an even coat.
Paint is used to decorate, protect and prolong the life of natural and synthetic materials, and acts as a barrier against environmental conditions.
Paints may be broadly classified into Decorative paints, applied on site to decorate and protect buildings and other objects.