Relay short circuit arc suppression
When the contacts close, if a short circuit exists in the load, the current will increase very quickly (based on the supply voltage and the impedance of the source and load and wiring (a loop). The contact does not close instantaneously, it may bounce a few times, and the pressure starts out very low, so a great deal of heat is produced right at the small contact surface. That heat will cause a rapid increase in temperature, vaporizing some of the contact material and it will often cause the contacts to weld together. 

The only way to prevent this from happening is to limit the current, and/or prevent it from rising so rapidly. Unfortunately, the obvious way to do the latter (a huge choke) will cause arcing when the contacts open, if they are carrying significant current. 

It is possible to make an electronic circuit breaker capable of opening into a short circuited load at the peak mains voltage. That would protect the contact but the seniconductor has to be rated for the service and there are other issues.