The dots on the guitar neck are there for reference
The dots on the guitar neck are visual references. They help you jump to a different part of the neck quickly, easily, and with accuracy.
For example, if you were playing Tunnels by the Arcade Fire, you might play a power chord on the 1st fret followed by a power chord on the 10th fret. The dots, being visual references, make a nine fret jump a bit easier.
Most manufacturers not only put dots on the fretboard, but they put dots on top of neck as well. This makes it super easy to see where you are on the guitar, even if you have no frets on your guitar's neck.
Really expensive classical guitars, like more than $2,000, have a dot only on the top of the seventh fret, and that's if you are lucky. Why? Because classical guitar is by-far-and-away a full-contact sport. You have to know what you're doing without a visual reference.
Dots are for barre chords, power chords, and lead guitar
Dots on a guitar neck are most useful when it comes time to play barre chords, power chords, or if you feel pulled to learn lead guitar.
In other words, if you are satisfied playing G C and D, don't worry about the dots. They won't matter to you unless you like the look of fancy fretboards.
The double dot: 12th fret
Likely, the most important dots on a guitar neck are the ones that you'd find on the 12th fret of the fretboard.
Why is the 12th fret important? Some of the answers are technical (fair warning):
- The 12th fret divides the fretboard into two visual halves. This helps lead guitarists visually navigate the fretboard.
- The 12th fret is an octave higher than the string's name. In other words, the 12th fret is where the string name repeats itself, but sounds higher. If this is confusing for you, don't worry about it. It's not that important.
- The 12th fret marks the absolute middle of any guitar string. This means that the string is exactly the same length to the bridge as it is to the nut when starting from the exact 12th fret. This gets more important if you are a tweaker and want to make your guitar as perfectly intonated as possible, but if you can get away with not worrying about this for the most part.
The single dots
The single dots that are usually, but not always, marked on an electric or acoustic guitar are the 3rd fret, 5th fret, 7th fret, 9th fret, 15th fret, 17th fret, and 19th fret.
As I mentioned before, you won't have much use for the dots on the guitar unless you are into barre chords and/or power chords, or if you're totally into learning lead guitar. If you are at this point in your process, then my suggestion is to use the dots as a visual reference. Let's talk more about this in particular.
How to use the dots as a reference
If you want to play a power chord on the 6th fret, use the dot on the fifth fret as a reference. Then, place your first finger on the sixth fret and play a power chord there.
If you want to play a barre chord on the 8th fret, use the dot on the 7th as a reference. Then, place your first finger on the 8th fret and play a barre chord there.
Lead guitarists will most often reference the 12th fret and divide the guitar into two parts visually. That way, instead of being intimidated by an entire fretboard, one only has to worry about half of it at one time.
Overall, the frets on a fretboard are used for visual reference. Unless you are a cage-fighting classical guitarist, you'll find that the dots on a guitar neck are useful for your aesthetic viewing pleasure as well as getting a little better at playing guitar.