Applying foundation can be tricky. Sometimes you want sheer coverage but at other times, you want medium to full coverage. How do you achieve control over your foundation? This is where the foundation brush comes in. The two most common foundation brushes are the stipple brush (shown in the middle above, from Trish McEvoy) and the flat brush (the last one, from Bobbi Brown).
If you want sheer coverage, use the stipple brush because its fibers are not densely packed, allowing you to spread foundation evenly and lightly with gentle strokes all over the face.
If you want medium to full coverage, use the first brush from Shiseido, whose bristles are thick and dense, therely depositing more foundation on your face per stroke.
The last brush, a flat one from Bobbi Brown, is often used for sheer to medium coverage. I don’t know why these flat brushes are everywhere but I don’t like them because they drag the foundation around your face and cause streaks. You have to work harder and longer to get the streaks to disappear.
Therefore, I suggest you get only the first 2 brushes: the dense Shiseido brush and the stipple brush (which you can buy from Trish McEvoy or other cosmetic brands).