In many cases, it can be useful to have bits with certain types of functionality arranged either consecutively or regularly spaced. Suppose a 32-bit microcontroller has three widgets A, B, and C, each of which has five associated control bits V, W, X, Y, and Z. If the five bottom bits controlled the functions for A, the next five for B, and so forth, then if a future enhanced version of the chip added another function U to each widget the control bits for those functions would have go in some "odd" place relative to the first five. If the bottom three bits represented function V for each widget, the next three bits function W, etc., one could continue the pattern with a new function U, but adding a new widget would require its control bits to go in an "odd" place relative to the first three.
Suppose instead one uses bits 0-4 for widget A, 8-12 for widget B, and 16-20 for widget C. One could then easily add up to three more bits for each widget while keeping the bits for each widget together, and could easily add another widget while continuing the same pattern. Even if one added a fifth widget, one could spill that into the next word address. Code which wants to access multiple widgets at once would have to deal with the fact that there's now more than one group, but code which just wants to use one widget may be able to use byte addressing.