Mathematicians need a lot of symbols for variables, constants, vectors, operators, sets, spaces, and a lot of other objects. So they use small and big Latin and Greek letters, calligraphic letters, or write upright, italic, or bold so that they can be distinguished from each other in the same document.
In a lecture, when writing on a blackboard or a whiteboard, it is difficult to write bold letters. So, double stroke letters showed up. Well, in our documents, we still can simply switch to bold, on the other hand double stroke letters for number systems are already very common. Furthermore, a typographer may like this, since it doesn't destroy the grayness of a text, in contrast to bold symbols.