One example of why cleanliness is important is if you have a dust allergy. Within the home dust mounts up incredibly quickly and that can leave an allergy sufferer in a lot of distress. So we need to clean to get rid of the dust.
Another example could be if we didn't tidy our homes and bits of food were left everywhere going moldy. It would attract pests into the home which could potentially give you health problems as pests usually are dirty and they can pass various things onto you.
Overall cleanliness is important for our health so that we do not get ill.