Of course, if you're a medieval scholar, the answer to that question is easy -- you've got to be able to read the language to understand the primary documents of your field. If you rely on translations done by others, you're taking a lot of very dangerous leaps of faith; heck, some scholars won't even rely on the transcriptions of manuscripts by others, let alone the translations of other.
Most people, though, aren't professors, so the above doesn't really apply to them. Nor am I a big fan of the "for the love of learning" argument. Yes, for the love of learning you should learn something, but why Old English? Why not Old Franconian? Or the history of Uzbekistan? Or Fortran? Or animal husbandry?
Of course, non-specialists will sometimes decide to study Old English because of reasons peculiar to them. For example, I've known people who studied for these reasons: they loved Tolkien, and wanted to understand more of the references in his books; they were assigned The Dream of the Rood and were so struck by its beauty that they desperately wanted to read it in the original; they were medieval reenactors; they really liked the professor teaching the class; they needed an upper-division class and it was the only one that fit their schedule; etc.
I've found three general and very pragmatic reasons, though, that a non-professor might decide to study Old English -- reasons that most people haven't considered:
- You are or plan to be a high school English teacher. The two reasons I hear from high school English teachers for not studying medieval texts are: 1.) the school's anthology doesn't include them, and 2.) the teacher doesn't feel confident in pronunciation of the original language, a short recitation of which is generally part of a good curriculum. The first reason is, I think, a result of the second -- no supply because of no demand. Those teachers who took Old English wowed their colleagues and established tons of authority with the students when they confidently read the opening of Beowulf or the lines in which Grendel first emerges from the mere. Most find Middle English (usually reciting the General Prologue of the Canterbury Tales in high school) quite easy on an intuitive level once they've studied Old English. For the high school English teacher, a familiarity with Old English, even with just a single undergraduate course, puts them in a league above their colleagues. Ditto, by the way, for teaching high school grammar, as will become clear below.
- English is your second language. I have a lecture I give to groups of ESL students and ESL instructors about how an understanding of the history of the English language generally and Old English specifically can help in learning English. I've never had a group that wasn't astounded by what they could learn in my 45 minute talk. So many of the difficult things for foreigners to learn about English -- the so-called irregular verbs, the bizarre exceptions to rules -- follow very regular rules in Old English. By knowing that heritage, they can understand why English works the way it does, and master it more easily. It also helps a great deal in learning vocabulary. Nearly all the approaches to studying English as a second language consider vocabulary building in terms of learning the greatest number of words possible for standardized tests, usually through the use of Latinate stems and afixes: for example, nation, national, nationalism, international, transnational gives you five words. If you look at a list of the most 100 commonly used English words in speech and writing, however, you'll find that nearly all those most commonly-used words come from Old English.* The words that you will actually encounter in spoken and written English, then, aren't the new-fangled Norman imports -- they're the ol' fashioned Anglo-Saxon words.
- You write or edit for a living. This is connected in a way to the ESL reason -- so many of the guides to style, when they suggest avoiding ornate constructions or pretentious diction -- these are just short-hand ways of saying, "Write in modern English using words and constructions out of Old English." These words and constructions that are the roots of our language resonate deeply with modern English speakers. George Orwell got this; you should too. Understanding Old English can help you become a better writer and editor of modern English, if you'll simply apply it.
*because is the main exception to the rule, which doesn't come into English until around the early 14th Century.