Voice chat allows leaders to better coordinate the search for the mobs. It also provides an audio way for people to know that a mark has been found, instead of constantly looking at the chat.
Finally, since the hunts section of TS can only be joined via an admin moving you there, it provides a nice indicator that most people in TS lobby need a party invite, a LS invite, and be moved over to the hunts section.

TL;DR
Dont bash it if you have no clue how it works