Some of my local public service agencies (fire/police) use sound at the beginning of the transmission (BOT) as a PTT ID (a.k.a. ANI-ID or "signal ID") giving dispatch the ability to see which unit number is transmitting. The PTT id is programmed into the radio by our local Kenwood radio re-seller upon request, matching the individual's or vehicle's assigned unit number. A brief search for ANI-ID and the UV5R yielded references to menu 15.
The BOT tone is quite useful for the following: Someone accidentally keys the mic and you know who it is. A weak signal is heard, dispatch is unable to copy the unit number, but they can see it on the radio console. You forgot who the last person to transmit was but you can see their unit number on the radio display and call back to them.
A particular fire agency in the area uses an end of transmission (EOT) tone which in their case appears to be a glorified roger beep. Searches lead me to menu 39, "ROGER" on the UV5R menu.
The EOT tone is controversial as some people believe "roger beeps" are annoying while others argue that it helps prevent units from transmitting before the other is finished as they get used to waiting for the EOT tone.