I'm assuming "benne" is optional here? What does it add to this sentence?
Yes, it is.
It emphasises the "insideness." It can also be used when there are several insides, for instance a hospital with a court a building. Then you could specify that you're inside the hospital building. There are just many situations when it's beneficial to use another word to make it clearer what case you're using, tbh.
Why in God's name would you teach optional positional pronouns before you get to the past tense??????
This is the most baffling and confusing course structure.
so that you could say any action with any pronoun from any location around you. But only in present tense.
Agreed. The sequencing / prioritization of skills is one of the many issues that need to be addressed before this course can graduate from beta.
You can also say "bent" instead of "benne" here. It sounds a bit more natural to me than "benne". I would use "benne" differently. I think it is a bit closer to being included in something. As in "A légy benne van a levesben". - the fly is in the soup. Or "Az adó benne van az árban." - the tax is included in the price.
"Benne" is optional, but if you leave it out, you should change the word order: "A piros alma az autóban van" sounds more natural than "A piros alma van az autóban". (in fact, the second sentence emphasises that it's the red apple which is in the car, and not the yellow for example)