Offset blades are designed with a fixed bend near to the tip. This is for improved access to valves, known to require clearance setting in restrictive engine bays. They are most commonly used for tappet adjustment but can also be used to check spark plug gaps.
Offset blades may be quicker and easier to position in between motor vehicle valves. They are often dual marked in imperial and metric units.
The offset design may sometimes be more of a hinderance than a help. Many clearances require a straight angle approach which cannot be achieved with offset blades.