Short answer: Because I want to, silly!
Long answer: I am an aspiring polyglot (formerly a frustrated one, now meeting with some success). Native-born expert in English, two years in German in high school (still retaining quite a bit;
Deutsch ist am besser); and I know a good bit of Japanese, hard way first (reading and writing first, the AJATT method).
And, of course, living in Atlanta, one encounters quite a bit of Spanish (both of my kids got top marks in their high and middle school classes, too).
Koine Greek was always in my sights, from the moment I started picking up words in the original languages, but it never seemed a real possibility until Orthodoxy ‘clicked’ for me, and when it did, and meeting my parish priest (native-born Greek Orthodox) was a natural and supernatural success, ancient and modern Greek were clearly the next languages.
(Hebrew was a possibility, until I found out about the Masoretic Text and saw far too many people of faith getting sucked into the seductive orbit of Rabbinic Judaism. Once I found out that the Church basically started out all-Greek from the get-go, Greek was the natural target.)
Having a church full of modern Greek experts and several Ancient Greek experts at one’s disposal is quite handy while simultaneously learning to soak up a different spiritual paradigm, and given that the services are roughly 50-50 Greek-English, I get plenty of sightreading practice every week. The font used in the church’s printed material is a decorative serif font that frequently trips me up, so it keeps me engaged during Liturgy.
My next target is Old Church Slavonic/Russian (recognizing Cyrillic was a side hobby before, anyway, due to a Russian acquaintance, but I had no spiritual motivation for it—reading Mumiy Troll lyrics really isn’t necessary for a serious Christian

.)
As I already know many of the prayers in Greek, it is time to begin swapping those out for Old Church Slavonic, so I can begin accustoming my eyes to parse the new characters, and my mouth to pronounce, the prayers that way.
It’s just a fun thing I do. I mean, you’re going to pray anyway, why not use it as a language-learning time as well?
It also keeps my mind engaged in the prayers while I am praying, instead of wandering off in space somewhere (the mind-in-the-
nous thing is very new to me, being that Protestants have a different, all-intellectual, head-first, take on human composition, and I am still not good at it).
In any case, I just remembered I have a trilingual English-OCS-Japanese copy of the Divine Liturgy on my computer I can copy-and-paste stuff from, so I’ll go do something more useful before anyone
I’m bragging and not just a silly nerd with a lot of time on his hands.

Peace.