it seems to me that the question is similar to what is the purpose of purchasing a device with the software preloaded onto it -- in both cases, it's handy to have someone else do something fiddly. Installing your own operating system is only a little more technically involved than installing bitcoin core and downloading and verifying the whole chain. And waiting for the initial block download to complete is kinda a pain in the ass. The only reason people do it is so they don't have to trust someone else -- but if you've already trusted them to install the OS on your computer, why wouldn't you also trust them to give you a copy of the chain?