Unfortunately very few of us ride with a motorbike or team car behind us ready to change a wheel, make a leaning out of the window adjustment or pass us a spare bike. This means that carrying a selection of cycling tools is a must and with some multi tools a tool kit is nearly what you get.
Some tools can all but strip down a bike and some are made just for pre ride checks and tweaks. There isn't really a right tool, some riders like to be able to service their whole bike with the bicycle tool in their jersey pocket and others prefer to get back to their mechanic, garage or front room workshop. Looking at your riding style, regularity and how adept you are with a bike tool in your hand should lead you to the right tool.