Carl is one of Gru's many minions, and he serves in a variety of general duties.  He also likes to imagine himself as a firefighter, and has secured a bullhorn and some flashing emergency lights to add effect and volume to his efforts at warning everyone, in Gru's secret lair, of any possible fire danger!  This usually brings the minions actually assigned to Gru's small fire brigade, swinging axes and dousing everything abundantly with water from fire hoses!

Of course, that repetitive "BEE-DO BEE-DO" Carl uses for alarm, through his bullhorn, can get old pretty fast...and not only for Gru, but even for his fellow minions.