An obvious question? Yes! Worth talking about? Absolutely!
Guitar intonation is the accuracy in which an electric guitar or bass can produce a fretted note and the most important issue with any instrument. Setting the guitar intonation is the act of adjusting the length of the strings. By moving the bridge saddles North and South the intonation can be adjusted. The reason is because the there needs to be a way to compensate for the thickness of the string. Also and the same with stretching of a string when played. The act of pushing a string down to the fret board after all, to produce a note always sharpens a note. Guitar intonation, done well takes this into account. For the purposes here I will focus only on electric instruments for now. However there is lots of knowledge overlap on acoustic instruments so feel free to apply it.
To adjust guitar intonation or bass or guitar, you move the bridge saddles. Look at an electronic tuner. Check to see if the open note and the fretted note on the 12th fret are on pitch. If the 12 fret note is “sharp” then move the saddles back or South. Check and repeat this until both open and 12th fret fretted notes are equal. Likewise, Check to see if the open note and the fretted note on the 12th fret are on pitch. If the 12th fret note is “flat” then move the saddles forward or North. As a result the 12th fret note and its harmonic are now equal in pitch to the same open-string note, which is exactly one octave apart. Accurate guitar intonation is critical to pitch quality. Pitch quality is essential to “in tune” playing.
Wouldn’t you want to present your musical talents in the best way possible?
Poor pitch quality = “out of tune” notes which in turn = poor musical presentation.
Of course, you do.
Furthermore, it is not necessary for a guitar player to know this at all. It is essential however that the guitar possess this in-pitch quality and maintain as close to perfect intonation as possible. Buying on looks alone can be very disappointing. If however, you like an instrument for the looks, buy it. You probably realize that you will need to have it worked on to get it playable. Good. It’s best to speak to a Luthier/tech or guitar builder previous to any purchase. It’s like to asking a mechanic which car he would recommend before you walk into the show room to select one. You can benefit from his first hand knowledge and experience and not have to go it alone. After all, the mechanics know which cars breakdown.