It is a test required for admission into law schools in the US (that are approved by the American Bar Association).
The LSAT helps you get into US Law schools and then, after writing and passing the BAR exams after Law school, get into the Law profession.
Even though there is a proliferation of Lawyers in the US, the credentials give you a head start into becoming a revered Lawyer in other jurisdictions. In this case, you will be required to write the BAR exams of the country in question to be granted the chance to practice Law there.