Because the build-up of fat and grease can cause fires and also the fumes can cause health problems. We recommend having your oven cleaned every 12 months or accordingly due to more use. Also keeping your oven clean can prolong your oven because some things you cook maybe slightly caustic, salty meats, etc.
Please be aware final cost of oven cleaning is given before commencing work.But we have pictures and prices on this website to help you with choosing your situation. If you are NOT sure of the price please ask.