Because the ip could point to the server but the hostname is merely pointing to a domain which could be a subdomain which could have some other ip all together.
A domain has 220.127.116.11 as ip (example, is made up number). A subdomain (meaning it has the same principal hostname and then something added to it) could have altogether a different ip and to be therefore in a different server.
Also domains can also make it so you can only enter into them through their designed places for security reasons, for example.
You need to think about it as minecraft buildings made of different blocks.
Edited to add a thought to the questioner: You know there is content which is volatile and in differnt parts right? For example, peer to peer, bittorrent etc. There are even botnets using peer to peer being nowhere and everywhere at the same time, and websites which shows content through peer to peer too.
I can only hint the answer you want.