Why in gods name do?
Answers:
alot of rebuilt parts have a core charge and the old one has to be returned for a core credit !!
'Cos most of them are men.
because they belong to you and to prove they have changed the part
You will be very surprised if they were covered in lipstick and whale vomit wont you?
To prove they replaced those parts!
probably try to fit them then decided it was the wrong part.
normally when there is a surcharge on them, or if the parts place has tried to shaft you with a 2nd hand part.
Would you know if the parts had been changed if they didn't, or would you trust them to have bothered to have changed the parts they have charged you for?
The 'motor trade' is dogged with mis-trust and bad publicity, mechanics sometimes return 'old' parts to be clear to customer what has been changed (and why).
If parts are unwanted because you supplied them and they were not needed, be aware that parts do get 'dirty' when fitted. You should be grateful it's not you that has to fit them most of the time!
Also, there is a new issue these days regarding the environmentally friendly disposal of some items, that cost the mechanic lots and the public nothing; if you dispose of your own parts you pay nothing for disposal.
there there poor old mechanic
just like the old unwanted parts the mechanic is covered in oil
as for the bloody copper slip he may have cut his finger
The answers post by the user, for information only, UKQnA.com does not guarantee the right.