The purpose of the sauna buckets and ladles is to hold clean water used to pour over the scorching sauna rocks to create wet steam.
The water poured on the stones turns a dry sauna into a wet sauna; it will produce a sudden sense of heat and add humidity to the air inside the sauna. In a traditional Finnish sauna, you can have buckets full of hot and cold water inside all kinds of a sauna used to create the steam.
Traditionally the saunas didn't have running water nearby, so they used buckets and tubs to keep it.
Some beginner sauna bathers also used the sauna bucket to pour freshwater onto themselves for an immediate cool-off.
The material of wood bucket is white pine or Cedar.
It's your call. But the type of wood you want for your sauna should be high quality and resistant to heat, moisture, rot, and warping. Quality spruce and cedar are good choices. Experts tend to recommend pressure treated wood.