So if I want to get rid of these regions, first delete joins them and only with second delete I get rid of them? For what is an overlapping ( of regions) good or is just an overlapping a glitch?

Reread the previous post -- the Regions are not being joined when you delete the top Region, you're just seeing sections of longer Regions that had been partially covered up. If you want to get rid of any Region just select it and delete it. I don't know how you got those Regions to overlap if you weren't editing at some point with the Command key down, so if you don't have a musical reason to overlap Regions then I guess just don't use that shortcut when editing. If something else caused them to overlap, well, I don't know what that might have been -- Command-editing was the only way I could get it to happen.

What musical reason needs to overlap regions? It could be I used command, as in fact I don't remember how I got these regions. I just practicing different things.

Maybe overhanging notes that ring out across the barline, or whatever..? But if you don't need to have overlapping Regions in your music then there's no reason to have them; if you do, now you know how..

In which music style are overlapping regions in use? I'd like to make pop (disco, dance, maybe beat) or some instrumental.

If in the course of recording you end up with Regions that overlap, then that's fine; if not, then that's fine -- there's no "style of music that uses overlapping Regions". Just record the musical parts you want for a particular piece of music and let the Regions fall where they may..

If I have a region overlapped with another region and both they have notes and these notes sits on the same place where overlap occurs, do I listen to both notes of both regions at the same time then (in case of an overlapping effect) ?

It depends on what kind of track it is: If you have overlapping MIDI Regions (on a Software Instrument track) then yes, you'll hear the notes from both Regions; If you have overlapping Audio Regions (on an Audio track) then no, when an overlapping Region starts it'll take over from the previous Region, and you'll only ever hear one Region at a time.

Then overlapping regions are bad. As I create music by melody, regions would be listening time by time as normal melody. If the additional notes occur, not fitting to the melody, I don't need them, right?

Overlapping Regions are not good or bad, but if you intend there to be overlapping notes it would probably be better (less confusing) for you to have them on a different track and avoid overlapping Regions. And if you have notes that don't fit to your melody, you can always delete those notes in the Piano Roll Editor.