One reason is that more people will be willing to move somewhere they understand the language. Sure you can get by in many places without knowing the language,

One reason is that more people will be willing to move somewhere they understand the language. Sure you can get by in many places without knowing the language, but if you're going to live there for any significant length of time you're going to have to choose between learning it (a time commitment) or not fitting into society. Some people are happy with one or the other of those options, some people wouldn't be. You also make moving somewhere more complicated. When I moved from UK to Paris, I was fine with work, with socialising, but my French was (and still isn't) nearly good enough to deal with government paperwork, appartment searching, reading up on tax law... etc. Sure, this problem can be solved either by the country making it easier for forigners, or by people moving paying for somebody to help them in these areas, but again its another barrier and/or additional cost. There's also the matter of local work-related communications. Anything from taxis to tax offices, courier/mail services, etc...