Paint Roller is an ideal object for big projects such as walls, ceilings. Roller holds more paint than a brush in one take, allowing the paint to distribute evenly in a shorter time.
Brushes, on the other hand, focus on details like trims, corners, edges, etc. Using a brush gives you more control in painting, in which you can adjust to how you paint to create the texture you prefer. Moreover, there is a variety of painting brushes. You have the right to select which one suits your needs most.