My guess is
Only a few inches tall and slender. Billions of them- everywhere. Homes, offices, restaurants, airports even airplanes.
Glow sticks frequently appear in survival kits, which (depending on location) may be as widespread as to appear in restaurants. Billions is probably an overestimate though.
They just lie there until called for duty! And guess what? Their job hardly lasts for a few seconds and that is it! They don’t hang around. They are done for good.
Until activated, nothing much interesting happens. Although they do last longer than "a few seconds", they don't last an extremely long time. They also can't be "reset", and so are done for good.
But I must say they do a good job. Work in harsh environments. Very hot or cold.
Regarding harsh environments and good job, see above point about survival kits. I'm not sure about temperature ranges (very hot or cold is rather subjective).
They unite diverse groups and bring harmony.
Not sure but they do often appear at parties and such, and I'm fairly sure diverse groups would unify in a survival situation.
The result of their work can be very pleasing.
Who doesn't like looking at the pretty colours?
Edit: Could also be
These can last significantly longer than a few seconds, and some can be recharged. Also somewhat shorter than "a few inches".