I took Latin and there were no articles. I took French, and, voila, now there are articles. When did this happen and why did it happen? I can't think of a plausible reason we need them if the Romans got along alright without them.
Edit. Took out 'Greeks'
It is an evolution in the vulgar descendants of Latin in the Medieval period in order to make it more clear whether it is 'a tree', 'the tree', 'one tree', 'that tree' etc. While it's impossible to pinpoint a date of course, illa / ille was used in Medieval Latin to mean 'that' or 'over there', basically to indicate specificity of an object. In the Old French of the Medieval period illa was rendered as la.
You might consider posting this question in r/etymology as it’s a whole sub devoted to the origin of words, though I’m sure someone here might be able to answer