What are the benefits of practicing under a teacher vs. practicing alone? Question is in the title.

I was wondering about the benefits of practicing with a teacher vs. practicing alone.

By alone i mean when one has a great deal of time to practice and no worldly responsibilities such as a marriage, children, being the owner of a car, house, flat etc. meaning that one can then spend a lot of time on studying and practicing the dhamma.
If one has found a good teacher who is willing to teach, nothing like it. Unfortunately, good teachers are rare, and most wander alone.

It's like climbing a mountain without a guide. Sometimes we get lost, wander around a lot, maybe learn a lot of things in the process, but also get hurt by falling down. Most times the learning is not really essential to climbing the mountain, it can sometimes feel like it is impossible to continue, and other times one can think, "I have come so far, I can't turn back now".

In the modern age with so many dharma books and teachers online, one may not always need a direct teacher. However, one can get confused with a multitude of approaches and schools.

It's a risk, but it also protects from getting locked into a dogma or school. This can happen to anyone who finds a teacher or school at an early age and never encounters other ideas. The Dalai Lama (in his book "Toward a True Kinship of Faiths: How the World's Religions Can Come Together") has said that until he visited the Theosophical society in Chennai and met people of other faiths, he was pretty certain only Buddhism had all the answers, and every other approach was wrong. 

Ultimately we make do with what we have. We don't abandon our practice merely because we can't find a good teacher, we do it the best way we can. 

If we accumulate merit by helping others it will lead to better guides, books and teachers - dharma will find us.