the impact site is the first playable area in pikmin 1.. where you encounter reds and learn the main mechanics fairly quickly..
theres like 2 parts here,, and a ton of pellets
you can grow like.. 300 pikmin in a day after you get the engine (the first part,, and "the most important")
the second part can only be collected once you get blues,, and is in a clam thing
usually they drop a pearl that makes like. idk 5 pikmin??? idk
theres also like a ton of pellets in the starting area..
just. a place to go if your out of a certan type of pikmin