Servicing - time v's mileage

[Turbofree]

My car has always been serviced every 12 months regardless of the mileage. For example the last service was done within 12 months having covered about 3,500 miles.

Now this may be a stupid question but what benefit is there to get it serviced after 12 months as opposed to 12,000 miles?

[pdv6]

Oil degrades over time...