A good example of why you would use aliases is if you own a business named "My Example Business", and you want to register some domains. It's common to buy the myexamplebusiness.com, myexamplebusniess.biz, myexamplebusiness.net, myexamplebusiness.us, etc. to prevent your customers from becoming confused or someone setting up a really similar domain name and maybe stealing your customers. You could just add all those domains as aliases to your main domain and if someone visits myexamplebusiness.com or myexamplebusiness.us they are shown the exact same content without having to copy all of your files to two different domains.