Not aware of any specific usages, but maybe they're used as meta-sets. e.g. "consider a set of numbers 𝕏 and a set of numbers 𝕐 ... etc, which can't be done with the defined letters.
Unicode originally only had the double-struck letters for the defined sets ℂ, ℍ, ℕ, ℙ, ℚ, ℝ and ℤ, but the full alphabets were later added at higher code-points with "reserved" gaps where the defined seven letters would otherwise appear.
(Complex, Hamiltonian quaternions, Natural, Prime, Quotient (rational), Real and Zahlen, a German word for counting numbers, for anyone wondering.
Quick edit: Zahl is cognate with English "tell" and "tale" which are both related to "tally", believe it or not. "Recounting a tale" didn't used to mean telling(!) a story, but instead literally checking a previous count, and developed the wider sense later.
Also, ℍ not getting "ℚ" because ℚ couldn't have "ℝ" is kind of funny.).