A lot of bacteria lives on those brushes, and you apply that bacteria to your face, causing breakouts, rashes, skin sensitivity and so on. Brushes used for applying wet makeup (such as foundation, concealer etc.) should be washed after every use!! Brushes used for dry makeup (powder, blush, eyeshadow, etc.) should be washes at least once a week.
Honestly, I wash all my brushes after every use. I have a lot of issues with my skin, and I wouldn't want to do anything to make it worse.
How to wash your brushes: you can use shampoo, liquid soap, bar of soap, or you can buy a brush cleanser (usually a spray bottle). Just make your brushes wet, apply the soap, and then use your hand to rub them clean. I wash my sponge this way as well. If you use a brush cleanser in a spray bottle, spray it on a dry brush, and then wipe the brush on some paper. Obviously, leave them out to dry.
That's how easy it is! Clean brushes makes your makeup look better as well. The same brush can be used for different things as well. If you have a brush cleanser available, you can easily use the same brush for different things without messing up your makeup!
So there you go! There are many reasons to wash your brushes!
See you on Friday ;-*