What Guitar Scales Are, Technically, And Why We Practice Them

Musically speaking, we have seen that scales are the "building blocks" of music. Many times in playing all types of music, we find ourselves simply playing scales, backwards and forwards, and in lots of other patterns. So it would seem natural that it would be a good idea to know how to play them, and it is!
If we think about scales in terms of technique, in terms of what it takes to actually play them on the guitar, we realize that simply because a scale is a string of notes, the simple act of playing a scale is quite a demand on the player. And the faster the scale, the greater the demand. In fact, for guitar players, scales are a whole lot more difficult than they are for most other musicians. A piano player only needs one finger to play one note, but a guitar player needs the co-coordinated action of TWO fingers (or finger and pick) to produce one note, and that has profound implications. As a guitar player, you would be wise to reflect on and appreciate this fact.
Almost all guitar students are unprepared for effective scale practice when the first scale comes along. The concept of truly independent finger action must have already begun to become a physical reality in the hands in order for scales to begin to be practiced with benefit, and not harm.
Segovia wrote, in his famous collection of scales for the classical guitarist "the study of scales will solve a greater number of technical problems in a shorter amount of time than the study of any other technical exercise." In other words, if you are able to successfully play scales, and get one finger after another to do what it needs to do to get those notes, then there are a whole lot of other things you will also be able to do. So this means that the study of scales is one of the best things we can do in our practice sessions to develop and maintain our technique.
And this is one of the main points I want to get across here: scales are, at the very least, a primary technical exercise for all types of guitarists. For non-improvising guitarists such as classical guitarists, this is, in fact, their main purpose. A classical guitarist does not need a million scales at his fingertips. Since scales are serving only the purpose of providing excellent exercise for the fingers, we only need to select and practice the ones we feel give us that exercise. In fact, in Segovia’s collection, most of the scales are simply the same finger patterns moved around the neck.
Of course, there is a world of things to know about the CORRECT way to practice scales so that our fingers actually do learn to play them well, and so that the practice of scales helps us technically, rather than hurts us. For far too many students, scale practice gets them nowhere. That of course, is a whole other subject which I have addressed in other posts. At the least, you should realize that practicing scales successfully is a complex matter, not to be underestimated, and not to be undertaken without a lot of effort to understand HOW to practice them correctly.
With this is mind, we could interpret Segovia’s statement to mean "if you can figure out how to learn scales well, you can figure out a lot of other technical challenges that playing the guitar poses".