Old trams in London had no auxiliary power systems on the vehicles, such as hydraulics or air for brakes (nor even a 12 volt low voltage supply), and so everything had to be done, if at all, at the full 550v line voltage. The controller had typically about 8 forward power notches, four series and then four parallel. Back past the 0 point in the opposite direction were braking notches (rheostatic brake), maybe two, which operated by essentially reversing the motor connections to make them generators, producing a current instead of using it, which was then dissipated in the normal power resistances. Beyond this, further notches caused the magnetic brake (if fitted) to come into play as well, clamping the magnetic shoes to the rail.
The electric rheostatic brake falls off as speed does, so for a final stop another brake is required. The mag brake has the opposite characteristic, and is more effective at slower speeds, so the two together are complementary. Note that none of this has used conventional brake shoes on the wheels. All trams had these as well, applied by a large wheel or handle.
London went extensively for the magnetic brake, Glasgow was more modern and installed air compressors and used air brakes instead on brake shoes, even fitting these to older vehicles. Manchester apparently had neither, and until the end of trams there in 1949 pulled up at each stop by the driver hauling on the handbrake.
Electric brakes are fine but depend on the line voltage being available, so if descending a hill and the power supply fails (or, for trams, the trolley pole comes off), the brake is unavailable, and therefore you do need full braking ability by another means.