I can choose assign a ip for a domain, nut I don't what is that use for?

Well, traditionally you would require a static IP to a site but with the amount of useable IP addresses dwindling and the advancement in the HTTP field, you can opt to Share IP addresses. The web server and browsers are smart enough to determine where the request is going and get the right pages for you.

So for most purposes, a shared ip would do the trick. However, if you require to set up SSL certificate for that domain name, you will be required to have a fixed IP address to it.

For all other purposes, use the shared ip. You do not have to worry how many sites are sharing one ip. the ip does not have any limits on that. The issue is more how well your server can handle the load.

Thank you!!