Annual mileage is an estimate you gave once
Carriers rate how far a vehicle travels in a year, and that figure usually comes from a number the applicant guessed at the original quote. Work patterns change, people move jobs, households go hybrid. A stale number can cost money in either direction, and nobody audits it unless you ask.
More exposure, not worse driving
Two identical drivers with identical records are not an identical risk if one travels twice as far. The extra miles are simply more chances to meet somebody else making a mistake, which is why limits deserve more attention than the clean record suggests.
Commuting is not business use
Driving to one regular workplace is commuting and is covered normally. Driving between sites, carrying goods or being paid to be in the car is business use, and personal auto forms exclude it. The line matters most to people who think they are just commuting.