Harvesting Tools (aka Pickaxes) are tools used for gathering resources in Fortnite. While pickaxes can be used as a weapon, dealing 20 damage per hit, they are better used to gather resources. In Chapter 2: Season 1, harvesting tools got a new look as well as gliders.
In Fortnite: Battle Royale, the harvesting tool functions in almost the same way as it does in Fortnite: Save the World. Players are able to gather resources with it, and it can function as a last resort weapon that deals 20 damage if need be. It used to do 11 damage in Season 1. It changed to 10 in Season 2. Then it changed to 20 damage in Season 6.
One difference that the Battle Royale pickaxe has from Save The World is that players can purchase cosmetic harvesting tools for it, making it look and sound different from the default pickaxe. These cosmetic skins offer no advantage in game play, same with weapons and skins. Season 9 also has introduced dual-wielded pickaxes, a pair of small harvesting tools. As of Update 10.20, these cosmetic harvesting tools can be used in Save The World. If you purchase Save The World as of now, you get the Save The World pickaxes.
Harvesting tools are generally available either through purchase with V-Bucks or as a reward from achieving tiers of the seasons of the Battle Pass. They can occasionally can be obtained by challenges as well!