Our long-term goal is to learn the entire neck, but right now we want the biggest bang for our buck. Most people are going to get the most usage out of the notes between the 1st and 12th frets on the E and A strings.
The most commonly used movable shapes, the E and A string barre chords and power chords, have their root based on the E and A strings. In addition, three of the five pentatonic scale patterns and five of the seven diatonic patterns have both their Major and minor roots on the E and A strings (the remaining two of each have either one or the other on these strings as well).