A dirty oven can be dangerous and unpleasant.
Over time, a well-used oven that is not regularly cleaned will build up grime, and grease. Spillages become cooked-on carbon. Dirt and grease get into hard-to reach places.
The biggest safety risk is fire. The Fire Safety Advice Centre say "the kitchen is the single most dangerous room in your home" and that the same problems cause fires time and time again. They list a dirty oven, hob and grill as one of those problems. They say "a build-up of fats and bits of food can start a fire". Seven thousand people are injured in kitchen fires every year. That number doesn't include deaths caused by kitchen fires. And nearly two-thirds of all domestic fires happen because of cooking.
Food and grease spilled in the oven and on the inside of the glass door will continue to burn every time you use your oven. Smoke will come out of the oven when you open the door. When you inhale this smoke, it could be harmful for your lungs. If your extractor is full of grease and grime, it won't take care of this smoke for you, either. A dirty extractor fan can also be a fire risk as it impairs airflow and makes the fan overheat.
A lot of grease in your oven will also reduce its efficiency, meaning it will take longer to cook food whilst also wasting energy. This could be dangerous when roasting a chicken to time guidelines, as it’s important that the meat is completely cooked.
Aesthetically, food cooked in an uncleaned oven, especially baked goods, will start to taste of the carbon-based smoke and fumes. Your kitchen and eventually your whole house will start to smell of grease and smoke, too.
Having your oven professionally cleaned on a regular basis will prevent the build up of grease and burnt on food and keep your oven in good, safe working order.