Why do I have SyphonInject on my Mac?
Is this something that comes preinstalled in a MacBook?
No. It is a 3rd party app.
Is it okay to completely remove it?
Yes, it is okay to remove it if you don't use it. It must have been installed by the previous owner.
If this MacBook is a shared computer and it's possible that some other user may be using the app, it's safe to leave it.
Further relevant discussions on Web:
SyphonInject what is it
Is Syphon Inject a "safe" program that's used in OBS?