diagramming sentences is useful when studying other languages because it forces you to realize how the parts of our languages work together and how that might be similar to or dissimilar to constructs of the same idea in other languages.

In English, you telephone someone, but in french, you telephoner to or at someone. In english, you might like to telephone, but in French you would like telephoner. However, in English, to is a preposition, so realizing that it is actually part of a conjugation is weird, but when you realize that you don't "like to [do]" but rather, you "like [to do]", replacing the appropriate infinitive verb in French becomes trivial.

The diagramming comes in handy as a visual aid to keep the relations and flow of the phrases in order!

qwerty's calculus would be very useful to a computer engineer trying to write algorithms for multivariable functions

I use everything I learn. ///shigabooks.com/// This comic is an example of how seemingly useless information can uh, well you'll see 8)