Over time, pet hair and skin accumulates within carpet pile and the edges of upholstery, and can be a significant aggravator for a wide range of allergies, including asthma.
These contaminants can be difficult to remove with domestic vacuum cleaners. In fact, a standard domestic vacuum is likely to worsen the situation due to inferior filters, helping to broadcast allergens into the air.
Oils contained within animal fur will also gradually soil upholstery, leaving discernable darkened areas. Certainly, most tenants with pets will find that a landlord will want proof of a professional clean as part of a tenancy agreement.