Historically, drawing bitmaps has been greatly optimized in computer programming. Most operating systems and graphic environments have strong optimization when dealing with bitmaps and graphics elements. This factor, probably in conjunction with the need to accurately replicate visual assets of several target platforms, led to the introduction of bitmap styles in FMX.
Basically, this means the application UI (or part of it) is the result of the combination of different bitmaps or, more often, parts of a large bitmap that acts as a kind of palette. Having a single bitmap is handy in terms of editing (for example, with an external graphic editor) and you have a chance to highly reuse the same parts of the bitmap in several components.
If a vector style is more similar to the composition of FMX objects (primitives, vector graphics, and other sub-compositions of them), a bitmap style usually consists of a composition of style objects describing the portion of some...