Why should you learn the skill of guitar tuning? Because music is sound! The better your guitar sounds, the better your music sounds.
When you first learn to play guitar you develop the habits that make you a success or failure. Tune your guitar every time before you play or practice, and any time you think your guitar may be out of tune!
If you play guitar untuned you just won't sound good, and it'll be impossible to develop an "ear" for pitch. After you play for a while your ear will grow accustomed to proper pitch, and you may be able to tuneup with your judgment alone.
Pitch and rhythm are the foundations of music, and the rest is built upon these using knowledge for bricks, and practice for mortar!
You don't have a tuner handy, and your guitar is out of tune? This is the standard method of tuning without using a tuner. Tune by Comparing Strings.
You tune your guitar, then it goes out of tune? Or, you're guitar is well tuned but plays out of tune? That's happened to me through the years. This section can help you solve those problems. Solve Tuning Problems