When replacing component parts, why do manufacturers recommend replacing the existing mounting bolts with new bolts?

I’m not sure about all of them, but i know some bolts are made to stretch when fitted correctly and need to be replaced once undone. I’m no expert here, so don’t know what they are called.

There will probably be other reasons too... like taking more money off us.... 🤣

 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

imho many are sold with a thread lock compound already on the bolt, so reusing is fine just use an appropriate thread lock. Obviously dont do this with stretch bolts and make up your own mind about some high stress bolts. Personally I normally reuse all bolts except very high stress bolts on some parts of the suspension.

David Range Rover 4.2 Supercharged 2006
Ford Focus EcoBoost 1.5 2017
MG TF 2003

Previous Cars of note
Land Rover Discovery 2
Jaguar Mk2 3.8 - Company car
MG Midget 1974 - Concours

To add to the above comments, 'Stretch bolts' I have encountered have always been for the engine head(s), due to the composition of the metal they stretch to a set measurement at a pre-prescribed torque. ( Or turning angle )Once used they will basically not stretch to the correct settings again, due to the loss of elasticity (ductility )hence they are single use only. Generally thread lock is not used with stretch bolts unless the makers specify it's use.

On large ( Marine ) engines studs are stretched using hydraulic jacks, with up to 1600 bar pressure then the nut is tightened by hand, checked for no clearance with a feeler gauge and the hydraulic pressure released. The elasticity ( or stretching ) of the stud ensures the correct tension is applied, but due to the metal composition, these are re-usable and the term 'Stretch bolt' does not apply.

In some applications, ie Brakes, bolts can suffer from excess heat, heating and cooling a bolt will alter the metals properties. If a bolt is heated and suddenly cooled, it will effectively be hardened, thus changing the ductility, making it less elastic and more brittle, thus weakening the bolt, so safety could be affected. One of the reasons they change these bolts, plus corrosion etc.

Bolts subject to heat on say an exhaust manifold will not cause a safety issue on failure, unlike on a braking system.

If you have read this far, then award yourself a large libation or two, and maybe even some tasty comestibles, AKA a real ale and crisps.

Considering they cost less than a cup of coffee I don’t even think twice and like putting new bolts on my cars when replacing parts.