This question is far too vague to stand, so I've removed it. If you want to re-post, please:

(a) make sure to specify exactly what it is you're asking: matter in what context - international law, diplomatic relations, what? and do you mean governments, national borders, constitutions, or something else? and

(b) ask yourself if this is actually a question about history. If it's about why "countries" matter in the present, then you've got the wrong sub: you'd be better off posting to a sub devoted to your own country, or alternatively /r/politics or /r/philosophy.