Serving the file over HTTPS is good because it means no one can do a man in the middle attack to change it, but it's not enough to be secure. If someone compromises the server itself the file could be altered at the source. The point of the checksum is to ensure that the file you're downloading is the file you're expecting. If you host the file in one place and the website in a different place it's harder for an attacker to change both the file and the website that reports the checksum, so you can be much more confident the file is correct. HTTPS on it's own doesn't give you that.