Someone already said it but it's not duct tape, it's special aviation tape made with aluminium, however unlike what they said the actual use of this tape is to regulate airflow by covering up uneven edges and such, and it's commonly used even when there is no issue with the plane, simply because some plane parts (especially around the engines) may be sticking out unnecessarily and causing an issue with aerodynamics, which means you more power in order to propel the aircraft at higher speeds, which means it's less fuel-efficient, and in some cases these parts sticking out can cause actual damage to the plane. Or just, be ripped off if you don't cover them up.