Yea, and I think you mean, and if so I agree with you, that you've thereby acquired some useful TRANSFERABLE skills. Not every sentence you learn to decipher in a new language has to be practical in itself to be useful as an exercise. Indeed learning to produce 'nonsensical', grammatically correct sentences is a useful skill in learning to master a language.
PS. I don't think the above sentence is 'nonsensical'. It's just that it isn't within a context that would explain it's true meaning. But using our imagination we could give it a context eg maybe someone who is looking at a question that is going to appear on an exam paper or in an oral exam. Or maybe a scientist or philosopher who is considering an enigma. Or maybe someone filling in an obscure application form. Or a teacher planning a lesson. Or someone trying to solve a riddle. Or.......?
Copied this from British council English learning website. It says - Must(n't) is usually used when the obligation comes from the person who is speaking. Have to and must are both used to express obligation. There is a slight difference between the way they are used. Have to shows us that the obligation comes from somebody else.
I agree. Compare "I must wash my hair". "I must write to John". (The speaker feels compelled by an internal feeling of self respect or guilt or obligation) and "I have to get 20 questions right to pass the exam" "We have to pay inheritance tax" "We have to call our teachers 'sir'." (It's an externally set rule or legal requirement).