Manual tuning is an important skill to have, if your electronic tuner is on the fritz, or if you want to make quick checks and adjustments as you are practicing and playing.
The manual tuning system will put your guitar in good tune with itself, so your notes and chords will sound right together. What manual tuning will not tell you is this important fact: are you in tune with the rest of the musical world?
When using the manual tuning technique, it is a good idea to have an external reference point for at least one note of the guitar. This could be a tuning fork, another stable instrument like a piano, or an online reference.