Ag = Education is in possession of a purpose, like you have a cat. It owns it as an individual agent. Tá cuspóir agam = I have an objective. But Education is not an individual deciding on its own purpose: it is given a purpose, it is delivered with purpose, it comes with it...
For 'an' I would venture that we are not just talking about any old education, but Education, a defined thus generic concept, as in The Education System...
I think to include that translation would be generous. At this level, it is probably best for course creators to generally discourage direct word-for-word translations, where they don't make sense. (The above translation, to be considered for inclusion, is pretty poor at best).
Choosing to include it would promote the idea that translating sentences word-for-word leads to adequate translation. I guess, maybe... "there is a purpose to education", but even so, in some cases, translation should be based on arriving at the meaning of the sentence.
Also, with this translation, there is the confusion/consideration to be given to the likely abstract idea of education in the Irish sentence (with the def. article) versus. specified education in English (with the def. article). in this English translation.
Though, I'm still on the fence as to the situations where 'an' is used in Irish, when compared with 'article use' in English.