It's a valid question. The internet has revolutionized the way we access and have access to information. Transcriptions to your favorite music are available quicker and more efficiently. Tabs for just about anything musical exist. There are a plethora of videos out there that will show you how to play everything note for note. If you look hard enough, you can find method books or transcription books available for free or little cost.
But there is still a need for the live, one on one lesson in the basement of a church in St. Paul MN. If you find the right teacher, it can even be a more effective use of your money.
You Don't Know What You Don't Know
This is the biggest reason why you need a teacher: you don't know what you don't know, but a good teacher will be able to find out. There is considerable value in having an intelligent individual ask the right questions to figure out what the next step is for you, and develop a plan of approach to get you there. That is what a teacher does.
Entertainment Value
It might sound funny, but a good guitar lesson is half entertainment and half education. Many teachers laugh at the notion that half of it is entertainment, but I would bet that group also has terrible retention, frustrated students, and 'just teaches on the side' so they can pay the bills.
Why is entertainment value important in a guitar lesson? At every age level, attention is held & retained the best when it's funny, enthusiastic, and keeps you on your toes. The same goes for a guitar teacher! I have known some world-class teachers who went unrecognized because of their lack of these qualities in their teaching routine.
Experience Is The Best Teacher
Remember that hard place you were in where you were looking for your first job but they required 5 years experience to be considered? Well how do you get experience when you haven't even had a job yet? A teacher makes that easier. They have experience. It's their job to share it with you. Learn from their mistakes.