It is in everyones interest to be for open borders, if they are done right.
Basically because they are a win/win for everyone involved.
And by open borders you usually mean for members of a customs union. Not for everyone.
There are no borders that are open for everyone. (Well, there are, within customs unions, but people from outside then would have to go through at least one checkpoint.).
If you only mean for wares and goods, yes - there open borders are in everyones interest as well, because it makes markets bigger and reduces costs.
That then is dependant on having trade deals in place (and not terrifs or taxes).
Thats a question that has a pretty definite answer to it, once you've worked out all the different instances of what "open borders" means.
It doesnt mean, that everyone can walz in. It never does.
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You usually dont see open borders between states (apart from economic and political unions such as the EU), because states usually want to watch over their own economies.
There are instances though (like in europe), where you need goods from other parts of europe (smaller countries) so ofter, that you get thinking.
Also - people from the outside complain, that they have to deal with 15 different product savety specs, and that makes the good more expensive, ... so - if as Europe you want to play in the big leagues. You have to make something like the EU. At least something along those lines - see:
https://ec.europa.eu/taxation_customs/history-eu-customs-union_en
If you are britain, maybe you think you dont need it. But then you fight for access to the common market claws out for more than two years anyhow... So... It benefits everyone involved.
Now as to how far other countries need to be helped within a common market, or a common currency union, those are still matters of political discussion within the EU.
If you hear a political party lobbying for exiting the EU they never mean the common market btw.
Usually. They never mean the entire phrase at all to be honest.