These are a brilliant idea. They avoid the corners of the head. That means they decrease the risk of damage and avoid damage that's already there. As a side benefit, some spanners can fit several, fractionally different sizes from different fastener systems.

• Undo nuts

• Undo bolts

• Hold a fastener head still

• Avoids damaging corroded shoulders

• Avoids already damaged shoulders

• Each spanner fits multiple sizes

• Always wear eye protection

• Consider impact protection