Among the many reasons to learn Latin the least important (although a sufficient reasons in itself) is to learn how to speak Latin.
A much better reason to learn Latin is to learn how to speak (and write) English!
Learning Latin is really nothing more than learning language itself. It would appear that apart from simply providing the Romans with the most effective language for governing the secular world, and the Church for governing the spiritual world, Latin was designed expressly for this purpose.
The celebrated English Author Dorothy Sayers:
It is the quickest and easiest way to gain mastery over one’s own language, because it supplies the structure upon which all language is built. I never had any formal instruction in English grammar, nor have I ever felt the need of it … the study of English grammar in isolation from the inflected origins of language must be quite bewildering. English is a highly sophisticated, highly analytical language, whose forms, syntax and construction can be grasped and handled correctly only by a good deal of hard reasoning, for the inflections are not there to enable one to distinguish automatically one case or one construction from another. To embark on any complex English construction without the Latin Grammar is like trying to find one’s way across country without map or signposts… (full article here)