Się and Non-Się

Why on some verbs is się added when it doesn't change the meaning of the verb at all?
Ex. Ukrywać, ukrywać się, they both mean to hide.
or zmienić, zmienić się, both mean to change.

ukrywać - hide something or sombody
ukrywać się - hide yourself, him/her-self (się = -self)

Okayy and what about zmienić?
change something
and change yourself? or what?

Same like for ukrywać

reflexive verbs are common in European languages (German, French, Italian, Slavic and of course Latin)
some exist in pairs (refelexive + non-reflexive), but some verbs only have the relxive form.

I am aware of this.