What are the benefits to maintain the nice urls for multi lingual sites? I understand that it has to do something with SEO, but what exactly?
If you have similar content in various languages, you can build similar-looking page trees in each language, differentiated only by the "language" string in the URL. You can put language-specific links on your pages linking to the "sibling" page of each page in each other language. So it's an organizational aid for you and it's also nice for sophisticated users who can then choose to go to the site in their favorite language.
Beyond that, it's just a naming convention. You can go with it or not, as you prefer.
So that search engines can index all languages. So that users can link to the page without losing the language. Because it is the only way to do it.
It is basically just an organisational aid.
You can just as easily organise your content into pages like site.com/en/blah
and site.com/fr/blah
.
If you maintain language options in subdomains, you may simply mirror your entire site structure into each document base. This requires minimal effort in maintenance, and is easy to create.
Also, by putting language options as part of the domain, as opposed to using cookies, sessions, and whatnot, you allow the page to be indexable by search engines, and also provide a permalink to a certain portion of your site that visiters may keep track of.
Subdomains also shorten your domain as opposed to get requests.