Long u closes the set of three basic vowel qualities, and the six letters ending with it are the frame the rest of the vowel section is built around. Its dependent sign hangs below the consonant like the short one, with an added turn to separate them, so this is another of the pairs distinguished by a small difference of stroke and another of the places where a hurried copy goes wrong.
In running text the letter is not common. Sanskrit has relatively few roots with an inherent long u, and most occurrences arise from grammatical formation or from the merging of two vowels across a word boundary, where a short u meeting another u yields the long one. That gives the sign the same character as several others in the table: predictable from the grammar, and therefore a place where a scribe who knows the language will be right and one who does not will guess.
Its independent form is correspondingly rare, since few words open on the sound. Like the other independent vowels it is nonetheless taught, written and recited in its place, because what a learner acquires is the table and not a list of what will be needed. The distinction between what a script can write and what a language happens to use is one the Indian tradition kept deliberately visible, and this letter sits on the line between them.