There are three main reasons to learn Koine Greek:
The Greek language has had a profound impact on our English vocabulary. Many of the most fundamental philosophies began or greatly expanded in Greek culture (including medicine, mathematics, metaphysics, etc.), and their terminology is still very much present in modern English today.
2) Classical phrases and texts
Koine Greek, the form of Greek used from approximately 300BC-300AD, was a very broadly used language. Learning Koine Greek allows students to read these ancient writers in their native language – including the New Testament writers and the early Church fathers.
3) Cognitive Development
The very act of learning Greek teaches students far more than Greek itself. It teaches linguistics, which is applicable in every language; this also leads into the study of logic and rhetoric. It also teaches critical thinking skills, since grammar is a very precise and delicate subject manner. In short, students learn to think clearly and analytically as part of the Latin process.