Like it or not the dealers do have a list of standard checks that they (should) carry out on every service and will strictly follow the schedule for whatever level of inspection / service the car is actually due.
They have access to the latest tech bulletins and recalls for your car, will carry out anti corrosion inspections which you'll need should anything rust down the line and you want to claim, should the car still be in warranty you will also have the least hassle claiming when the manufacturer has carried out servicing on the car )regardless of whether oem parts were used or not).
Lots of people also buy servicing packs which can make the cost more reasonable. Any other recourse you need to take for work you aren't happy with or failures is much, much easier through a dealer network than it would be with anyone else
Last but not least exposure to your buying market can decrease significantly if you try to move on a newish car with no dealer history - or an old, very complicated very expensive car with no dealer history.
There are a whole host of reasons why you might not want to use the main dealer too - price being one of them, but to be honest I've found that I can get the dealer down to near as makes no difference a delta to an independent I trust on servicing.
If I buy a car that already has a full dealer history I'm inclined to keep it up even if it's ancient. If it's already been serviced elsewhere and is over circa 6or 7 years old I'll usually go to either a specialist or a friend in the trade