Whenever you ask students to proactively do something, some percentage can’t or won’t follow through which causes you headaches. If you ask them to create a private repo and share it with you, 1) some of them will make it public by mistake, inviting plagiarism (or confusion about plagiarism) and 2) some will forget to share it with you, and you have no way of knowing that the work was done and you need to grade it. Classroom forces students to “accept the assignment” before they can begin, so all the submission are organized and have the sharing settings that you specify.
There is some momentum behind this workflow; Github created their Classroom Assistant downloader, and individuals have created tools, and all of them work with Classroom but wouldn’t work with ad-hoc setups.
Also the explicit Classroom object serves as a record, outside your personal account, of what work was submitted. This is helpful when multiple instructors are teaching together. And it decreases the “bus factor,” if for some reason you were unable to finish teaching your class, it is at least technically possible for a substitute to pick up where you left off.