The two main benefits to me are ones that nobody else ever seems to mention. Firstly, there are some skills (eg communication skills) which are so valuable in most of the situations we find ourselves in. The strategies used in communication are relatively simple; they're in most of the NLP books for example.
However, there's a big difference between knowing the strategies and using them well. It's *when* those strategies are used and how they can be used most effectively within your model of the world that determines whether you're going to be a master or merely above average with those skills.
Some trainers have learnt how to install skills in you that are are congruent with everything you are and want to be, to a level refined beyond years of book learning. Just don't ask me how they do it :-)
Lots of people who learn NLP settle for so little. Mostly that's due to the training they have. It's not easy to teach NLP in 2 or 3 weeks and I have a profound respect for the ability of NLP trainers who consistently help students go way beyond what they came for. These trainers are probably the most skilled people in the world.
This leads into the other major benefit of training. NLP is based on a simple set of fundamental principles plus an evolved body of knowledge of what generally works. The former can be learnt from books and I suggest you do that. The latter came from modelling or replicating what experts were able to do.
Modelling from experts is one of the best ways to learn anything. In demonstrating a model (way of doing something) that works exquisitely, they provide an explicit goal (being able to do it), plus the structure of how to achieve that goal.
Motivation is also very important, and one of the main motivators for me is knowing that I will be able to do more of what these trainers can do as I continue learning.
The last thing I'll say is that your first NLP will probably be more important than all the rest. It is something of an imprint experience and the trainers are likely to install an attitude towards NLP that will probably stay with you for life.