3 reasons why in the first few years of a vehicle's life it's important to use recommended parts, service intervals and trained technicians?

It’s not important despite what many people think.

If the car does not come with a prepaid servicing plan, and if it’s not on a lease (many leases insist upon annual main dealer servicing), then it’s entirely good enough to ensure that the car is serviced annually according to the exact same checklists as stated by the manufacturer, using *either* OEM parts or aftermarket parts to the same minimum specifications.

Main dealer servicing does tend to maintain a higher vehicle resale value. But the financial savings in getting the maintenance done at an independent workshop almost always comfortably offset any potential reduction in resale value.

Good independent workshops also often have access to almost all the same TSBs (Technical Service Bulletins) as main dealers, and certainly all of the critical ones.

The only cars where those maths don’t add up is for specialist prestige models like Porsche, Ferrari, Bentley, Rolls Royce, Lexus and higher end BMW and Merc models. Buyers of those absolutely expect the first owner to have remained entirely in the main dealer network. Electric vehicles also tend to need to visit dealers as independent workshops don’t have the specialised computer equipment and software needed to check and update vehicle firmware and software.

When I wrote “serviced to exact same checklists” I meant what that says. It does not mean a simple fluids and filter change and a wipe over with a wet cloth. Independent workshop standard pricing does not allow for “same checklist” maintenance, but as each car model has its own set of checks which may take longer of may take less time then they will usually already have prices worked out for individual models

Warranty, warranty, warranty. Car manufacturer might not honor its warranty if you booger it up with aftermarket parts and service.

Many times car dealers get recalls or PCM reflashes for common problems that are discovered on a new vehicle. 

1. To preserve the factory warranty.

2. See #1

3. See #1

Why do you need three when you only need the one: because all of that is generally included automatically and free of charge at dealerships under the vehicle's bumper-to-bumper warranty, making it pretty stupid to go out and pay for otherwise elsewhere.