That wasn’t done by Jane Street. This is part of the language from the start and was introduced by Leroy team at INRIA.

As to the point of studying boxing, well, it’s fairly basic compiler theory. It will be hard to avoid if you have even the slightest interest in PL.

Optimization of code is valuable in a lot of industries where time-to-complete or time-to-react is proportional to money spent/lost.

Most of their cohorts just use C++ for that. Several Wall Street firms wrote their own language back in the day. Until the business people figured out IT people just liked to write languages and tell happy stories. They are in the money making business, not the language writing business. Division of labor.

I spent 3 years at Morgan Stanley writing A+ and loved it. A+ is dead but kdb lives on, and is widely used at top hedge funds.