Is it necessary to have an associated SIM for using FaceTime and iMessage?
You must have a SIM associated with an Apple device to be used for iMessage and FaceTime.
[Edited by Moderator]
iPhone 15, iOS 17
No. You can setup iMessage and FaceTime using your email address (Apple ID). If you do this, then you will have no phone number associated with iMessages and FaceTime and these two services can be reached only by using your email address.
Axel F.