Why does the A320 have an anti skid system and how does it work?
It's very similar to Anti-Lock-Brakes on cars. 


The main reason aircraft have it is that a locked tyre will heat up, wear excessively and can burst (aircraft tyres are typically run at much higher pressure than cars or even HGVs). 
If you lock a wheel under braking, it no longer provides any steering. Aircraft might not be swerving around obstacles, but staying on the runway is more important than minimising stopping distance.
A locked wheel generates slightly less than maximum stopping force. An automated system can hold the brakes at the point where it's not quite skidding, and brake harder than a human trying not to skid. 


They work by monitoring the speed of each wheel, and releasing the brake on any wheel that slows down significantly more than the rest.

I believe that airliners are tested to check that locked wheel won't wear to the point where it bursts, but you can only do that once!