Or what must be under your seat to avoid any complicated situation...
In most cases, the original tools set is enough. Just check that you have a spark plug wrench, 2 screwdrivers (1 for slotted and 1 for philips heads), 2 or 3 allen keys (4-5-6 for example), the most common wrenches (8-10-12-14), wheel-nut sized wrenches and one plier.
No need to discuss about its utility...just forget it to need it !! In the set you have some plugs, CO2 inflator+cartidges, one cutter, 2 tools (one reamer tool and one plug installer tool) and, depending on the set, a glue tube. Maybe could you read the instructions and learn to do it in a relaxing situation.
You can find really small ones, which can easily keep all the time under your saddle. Really helpful to check pressure after using a tubeless repair kit, or wherever you want.
Everything is described on the manufacturer site http://www.sis.free.fr/. I must confess I did some hacking, because I downloaded mine on a cherry jam jar. It may anyway be very helpful any time you want to use sidestand on a doubtful ground.
Really small and wonderful to adjust your chain slack accurately.