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. Also, ℍ not getting "ℚ" because ℚ couldn't have "ℝ" is kind of funny.).