The Stim in Titanfall 2 will significantly boost your speed by doubling it and will heal you after a short delay after activation. The ability is very useful if you are in the middle of a firefight and need some health, or you need to get away from a bad situation, while also being effective as a combat asset, allowing you to get in close for maximum damage with a shotgun or a melee attack. It can also help you in rodeoing enemy Titans and riding friendly ones, if you need a battery or one of your teammates does. Being able to significantly enhance your speed in any case, it can get you out of trouble, and it can make trouble.
Use it for short range combat, like with shotguns and melee.
Use it to escape a bad situation or get into a firefight quickly.
Use it to assist you in rodeoing an enemy Titan, or escaping from said Titan after the Rodeo.