Acne is not only annoying at the time but can lead to long term scarring. Acne scarring is generally caused by moderate or severe acne, particularly where there are acne nodules or cysts. This is one of the reasons why it is important to rapidly and effectively treat moderate and severe acne - with tablet treatments if required.
Scarring reflects the body's natural response to injury - during human evolution it was important that any injury healed as rapidly as possible to seal a break in the skin surface that could lead to infection which in the era before antibiotics and modern medicine was potentially fatal. Whilst wounds in the human fetus can heal with no scar, all healing in the child or adult is associated with at least some degree of scar formation.
As for any cause of severe inflammation, acne lesions cause damage to the epidermis and the dermis with death of keratinocytes and fibroblasts and damage to normal skin structures such as hair follicles and sweat glands. This damage is repaired by growth and division of fibroblasts - the cells responsible for production of the connective tissue in the skin. This leads to the formation of tough bands of connective tissue that characterize scar tissue and can be contrasted with the delicate fibres that make up the normal dermis.