To reduce wheel speed in case of wheel slip, there's a provision of anti-spin brake as well in 3-phase locos. It provides a rapid, yet soft brake application to the control the spin of the wheels if in case the wheels start slipping.
Parking brake has nothing to do in this case. It is present only on some wheels of the loco. Like we use handbrake in a car for parking, a loco too will require some sort of brakes to keep it at standstill when it's not in use.
Parking brake provision...
more... must be present in conventional locos too.
Wheels start slipping or skidding (these two terms are used interchangeably) when the tractive effort or braking effort breach the adhesive limits of the loco. How can slip/skid occur when the loco is not in use? :P