The Helinox Powder Baskets are useful when crossing hard packed snow or soft ground. The large surface area helps prevent the trekking poles sink into the snow.
Helinox says:
Set of Powder Baskets for all Helinox Trekking poles.
For a lot of the time when trekking, the standard baskets that come with your poles are perfectly fine. If you're going to be encountering snow then these are ideal. These baskets will also help you traverse a boulder field, there is less chance your pole will get stuck between the rocks and potentially get damaged.