They were considered a complementary tool to sprites, which were sharply limited in both number and size by hardware. Blobs could have the same kind of effect, but with much larger objects and/or with a much greater number.
BOBs anyway required more processing power than sprites, because they required at least a memory copy operation to draw them on the screen. Sometimes three distinct memory copy operations where needed: one to save the screen area where the BOB will be drawn, one to actually draw the BOB, and one later to restore the screen background when the BOB is moved away.
Provided that the order of drawing is kept consistent, multiple BOBs can intersect their movements and overdraw each other without problems.
See also: Original chipset