Even when most of a wall’s surface is painted with a roller there still be one of more brushed used on the corner and edges.
The type of brush used will affect the look of the paint.
Flat brushes are fairly common for general purpose painting. The bristles are arranged in a square or rectangle shape.
Tapered brushes have bristles that thin out towards the tip. These are popular amid professionals. The brush is designed to give better control when painting. They give a smooth finish while still being rigid enough to remain straight.
Angled brushes have the bristles sloping from left to right (or right to left). This is useful for some edge painting. All interior painting will have a few edges.
The bristles of a brush make a significant impact on the finish of the paintwork. Fine bristles give a smoother finish to glossy paint (if the painting technique is good). Thicker bristles stay straight, even when holding more paint.
If the paint is matt or eggshell then the thickness of the bristles makes less difference as there is not meant to be a shiny surface.
Tapered bristle brushes are popular with professionals, giving the best of both worlds. The thin edges produce a smooth glossy finish; the thicker middle and base keep the bristles straight. This is the best option for glossy paint.
Natural bristles were once the only option. They are very suitable for oil based paints and produce a very smooth finish.
Synthetic bristles are made of nylon or polyester. They are good for water based paints, but not oil based paints which may chemically react with the synthetic material.
These synthetic bristle brushes can be tapered or straight. They are very long lasting.
Professional painters will use brushes to give the right edge to the interior roller painting.