Star Trek: 10 Things You Didn't Know About Combadges
7. Their Use As A Transporter Aid
As most fans know, Starfleet transporters are some of the most buggy devices in Trek. A passing ion storm, a thick atmosphere, and many other things can block the signal and make beaming much more difficult, if not impossible.
One of the ways Starfleet counteracted this was by locking onto combadges rather than peoples bodies. Apparently, combadges give off a much stronger and more easily detectable signal that can cut through distorsions.
There have been a number of times in Trek when this technique was used. For example, when a bomb was planted on the Enterprise in the Next Generation episode The High Ground, Geordi attached his combadge to it to help the transporter room beam it into space.
It can be assumed that combadges must have some sort of easily distinguishable rare material inside of them, or some form of high-powered energy emitter that makes it easy to detect them even with huge amounts of interference.