The guitar is in fourths on the lower strings because that's where you play chords. The fourth is the "longest" interval between strings that lets you play the major third/minor third diad intervals withing typical chords with "closely spaced" fret positions.

The top 3 strings are "respaced" to make it easier to move around through "all of the notes" to play the melody (or the licks) there. The guitar is a relatively large instrument, so the same principal as with the standup base comes into play. To easily reach the notes you need for what is normally played on the upper strings, the strings themselves are tuned "closer together."

For some uses, the respacing helps with the "modulations." When you use changes between major, minor, dim, or "fourth note" (6th, 7th) chords, you encounter "shorter" entervals (and especially adjacent short intervals), that also are more readily fingered with the spacing on the top 3 strings.

In typical tunes, the chord changes "move" in thirds, fourths, and fifths (intervals between the tonic of consecutive chords), so the jump from one chord to the next is facilitated by the same spacing that makes it easy to play the notes within a given chord. The single-note-shift that moves the chord to modulate between major/minor, or to add/drop a seventh, frequently involves two adjacent minor third intervals. Just as the fiddle strings in fifths don't permit the major third/minor third pair in a typical major chord (two notes are always on the same string) the strings tuned in fourths don't permit a "ready facility" in playing the minor third/minor third adjacent interval in a dim or 7th chord.

Same principal with "drop/open/slack tunings" etc. When you want to "do something different" sometimes you "tune a little different."

Fiddlers commonly use double-stops on open strings. The bow can only contact (for mere-mortal players) two strings at a time, and the open strings are a fifth apart. Unfortunately, moving to the next pair of strings jumps the "chord" by a fifth, and the tune probably went only a fourth. It is somewhat difficult to finger two adjacent strings at the same "fret", especially if you need to be ready for the next quick note, so fiddlers sometimes retune a string or two so that they can "double stop fifths" - the "characteristic sound" - somewhere up the neck without having to use the "dreaded barre chord."

John