When I want to say that something has just happened "very recently" I use present perfect because something has just happened and has a present relevance or because it has just happened only?
Present perfect is used when an action started in the past and continues to the present.
We have eaten.
might have occurred an hour ago. However,
We have just eaten.
means not too long ago/very recently we ate.
Not to be confused with
We have eaten, just.
which might meaning you very recently finished eating or had a very small meal almost incomplete meal
We barely had something to eat.
You can use either the present perfect or the past simple with the adverb "just" to mean recently or a short time before speaking.
The use of the present perfect and the past simple is dominant in BrE and AmE respectively.