The first player I ever saw that removed the covers from their humbuckers was Jeff Beck during the mid 60s while playing in the Yardbirds. Jeff removed the covers on his sunburst Les Paul to get the pickups closer to the strings thus increasing the output of the pickups. It also reduced the microphonic feedback in the pickups (Gibson didn’t wax pot the pickups back then). There is also an increased risk of pickup squeal as the amplifiers were getting much louder. As the amplifier wattage got higher and higher, players started to complain about the pickup feedback problems.

Many players still keep the covers on their old guitars like Billy Gibbons from ZZ Top. Removing the covers will give the pickup a little higher fidelity and allows the pickups to get closer to the strings that can also increase the volume of the pickup. If the cover is not properly potted with the pickup, playing at higher volumes will increase the risk of feedback in the pickup.