Besides being cheap insurance, a bike pump can be lent to your buddies who are out of CO2 cartridges. Size matters - longer pump means faster inflation; so get the longest pump you’re willing to carry. A full-length frame pump inflates fastest [also good for thwacking Rover on the snout]. and a high quality mini-pump can do the job and tuck away anywhere, including a jersey pocket.  For our rides, we always carry a Topeak pump.