Most people do not need the attenuator - the HF35C alone is usually sufficient for basic measuring.
The attenuator allows you to measure readings 100 times higher. If your HF35C is maxing out (over 2,000 microwatts per square meter) and you really would like to have an idea how high it is or perhaps right down the readings for keeping records and as evidence, then you will want the attenuator.
For measuring very close to sources such as high powered cell towers, smart meters, etc, then the attenuator can provide you with a reading when the HF35C normally maxes out. So it is a nice accessory.