Every day, no matter what you have been doing, whether you wore make-up to work or were outside all day without make-up, dust particles, sebum and skin scales will have attached themselves to your skin and prevent it from breathing. So, they have to be removed every evening.
If you have also been wearing make-up, your skin may react irritably. This is why it is important to cleanse thoroughly every evening to prevent blemishes and your skin ageing prematurely.