"She is a teacher."
It is a general 量词（quantifier, measure word）meaning a or an, but it is generally better to use more specific quantifiers, e.g. 一把尺（a ruler）、一间房子（a house）、一位老师（a teacher）.
Two points to note:
1. Chinese makes a lot more use of measure words compared to English, where one would usually just use a/an.
- Using 一个 may be technically correct, but it sounds less polite when it comes to a person, e.g. 一个医生 vs 一名医生 or maybe just shows a better command of the language when you use a more specific quantifier. :)
It is a classifier for the noun in question, 老师. Since teachers are, well... humans, the classifier for humans, 个 is used. Then, a number is used with it. In this case, 一 (1). In this case, it emphasises the "singularity" or the teacher. She is a teacher, not teachers, although the singular pronoun implies this already.
Because it is still used and apart of the language so learners need to be introduced to it at some point.
Languages (in most cases) aren't a designed and logical thing. Just like us and how we evolved to have vestigial organs, languages have things that aren't really necessary but still exist.
This is one of the downside in using duolingo for me, sometimes they just throw out things out of nowhere without explanation
like why they decided to add "一個" now, i know it's to emphasize on "a", but the way and the timing they delivered this sentence will confuse a lot of people. there is no consistency