SOUTHERN BORDER WALL OF USA

Why USA building a wall in its southern border, is actually a really good idea?

First of all, is it really a good idea for any country to allow any/all immigrants in?

What is the point of any country in the world having immigration laws?

All countries in the world must allow absolutely anybody/everybody from any/all other countries (or just USA must do that)?
Or, USA must continue applying immigration laws against everybody from all countries in the world, except South American Countries?
(Maybe just because South American People live closer to USA?)

Would it be really better for USA to try to select best & brightest possible immigrants from all over the world, or, just let everybody in (coming from its southern border)?

Also, how about looking at some (recent) examples of what other countries did/doing?
For example, why Turkey decided to build a (big & strong & durable) southern border wall, after taking in millions of Syrians?

IMHO, it is especially vitally important for USA, to build a (big & strong & durable) southern border wall, considering the current political/social state of USA!!!

IMHO, currently, USA looks to be in a highly polarized political/social state!!!
A Democratic President gets elected by the public & Republicans start screaming "IMPEACH THAT PRESIDENT" !!!
A Republican President gets elected by the public & Democrats start screaming "IMPEACH THAT PRESIDENT"!!!
(Instead of both sides respecting/accepting the fair result of democracy!!!)

IMHO, in such a highly polarized political/social state, suddenly millions of new immigrants coming in, can be the trigger of a new civil war in USA!!!

Imagine, suddenly a (civil) war happens in South America someday (like what happened in Syria, for example)!
Millions of people decide to go to USA (where else? (but, only if, they know there is no border wall to stop them!))!
Of course, consider how many people in South America leave in bad conditions, so they also would join in to the "caravan"!
Of course, a significant percent of the people coming in, would be criminals, no doubt, running away from the law of their own countries & looking for a whole new world of opportunities!

A USA civil war in today's world, would be umcomparably more devastating/destructive, from the civil war in 19th Century, no doubt!
Not just the whole USA would be destroyed, but the whole world economy (highly probably) would collapse!

USA is the biggest/strongest democracy in the world!
What kind of message, its total failure/collapse, thru a civil war, would send to the rest of the world?
What would happen to other democratic countries (which are in any level of polarized political/social state)?
How much stronger all anti-democratic regimes/parties in the world would get afterwards?
Would it really good/ideal for whole humanity to live under anti-democratic governments, maybe for all future times?

But, also, of course, not any/all border walls are good!
For example, IMHO, a northern border wall between USA & Canada would be a really bad idea!
(Actually, IMHO, USA & England & Canada & Australia & New Zealand should try to create a EU-like union someday, for their own benefit!)