They say it boosts confidence, develops leadership and inter-personal communication skills, helps with job interviewing skills, and, well:
[...] There is no instructor in a Toastmasters meeting. Instead, members evaluate one another’s presentations. This feedback process is a key part of the program’s success. Meeting participants also give impromptu talks on assigned topics, conduct meetings and develop skills related to timekeeping, grammar and parliamentary procedure. [...]
Part of learning to give "impromptu talks" is "Table Topics", which develops your ability ability to “think and speak on your feet”, when you have minimum or no preparation. It also aids in learning to effectively answer questions.
And of course, there are numerous Success Stories. Not surprising, since Toastmasters has been around since 1924. They must be doing something right. I may check it out.