If the operating oil level of the engine is consistently below normal, it will be necessary to use a longer dipstick.

If sludge buildup on the bottom of the pan becomes excessive, it will become necessary to shorten the dipstick to accommodate for the false oil level reading.

When changing the oil in an operating engine, it may become necessary to use a longer stick to obtain the exact location of the oil level at all times.

In most situations this would never be done.