If you don't clean and sanitise your equipment before brewing, the beer yeast will need to compete with other wild yeasts and bacteria that are in the air and on your equipment.
To make sure your yeast works as intended, it's important to minimise the competition (and avoiding anything you do not want growing in there) by cleaning and sanitising all equipment that will be in contact with your wort.
Not correctly cleaning and sanitising can result in off-flavours and infections in your beer.