In the beginning, when you slowly play on one string (it doesn’t matter which), you should try to play as cleanly as possible (with almost no extraneous sounds).
Playing on one string develops range of motion and introduces the guitarist to the basics.
Playing intervals is a completely different matter, as here we use two notes and two fingers, meaning control and clarity must be doubled.
What happens to your fingers when you play parallel scales? You develop a sense of balance, each finger begins to “fall” naturally on the right string and fret, and you begin to gain confidence in your abilities, thereby encouraging you to increase the difficulty of the exercises to reach a level suitable for a particular piece.
Playing parallel scales helps develop finger coordination, improves playing on individual strings, and provides a very good foundation for polyphonic playing (which, in fact, many people want to achieve but don’t know how).
Just like that!
Rehearse, develop and improve!
