Because it changes the way you think about programming. Taking about Lisp, Eric Raymond said:
"Lisp is worth learning for the profound enlightenment experience you will have when you finally get it; that experience will make you a better programmer for the rest of your days, even if you never actually use Lisp itself a lot." Ada provides a different kind of enlightenment than Lisp:
1) Ada has a powerful type system that lets you define exactly what values are allowed and how they should work:
https://learn.adacore.com/courses/intro-to-ada/chapters/stro...
https://learn.adacore.com/courses/intro-to-ada/chapters/more...
2) Ada has built-in concurrency primitives that very simple to understand and use:
https://learn.adacore.com/courses/intro-to-ada/chapters/task...
3) Ada supports design-by-contract programming to precisely define the semantics between different parts of your code. Design-by-contract is ideal for large codebases and helps to catch errors similar to unit tests, but within the code itself:
https://learn.adacore.com/courses/intro-to-ada/chapters/cont...
Even if you aren't writing code for heatseeking missiles, learning about Ada can make you a better programmer.