Bias-T gives the possibility to connect an amplifier to the HV contact of the detector. It decouples the the high voltage needed to bias the detector from the low voltage input of the amplifier. It is an essential part of any TCT system where both contacts, grounded and HV one, at the same time. Particulars Bias-T offers wide band width with almost no pulse distortion and can tolerate high voltages up to 2 kV (depends on the version).
Biasing detetors through Bias-T is many cases the only option, particularly when grounding or shielding of the detector requires chasis of the detector to be at ground potential.