They main point is that
bash is meant for interactive use,
dash is used for system scripts.
bash comes with whole lot of features, among which is line editing. You have specific shortcuts to delete a word, jump to beginning of line, etc. This is convenient for writing commands interactively. However that comes at a price of performance and boot up times.
dash on the other hand is meant for writing system scripts that are fast and portable, thus helping keep the boot times short, and on Ubuntu it is compiled without line editing, even without
vi editing mode. Thus, it has less overhead.
bash is capable of running exactly same scripts
/bin/dash runs ( if they are written with portable syntax, of course ), but
dash is just faster at doing these same things.
Note, of course, that users are free to change their login shell via
chsh command, and you don't have to stick with
bash if that's not what you prefer.