For we learned: In the case of a cask of terumah wherein a doubt of uncleanness is born,
That is, whether this food item for the priests was touched by someone ritually impure or not. If it was, it could not be used.
R. Eliezer said: If it is lying in an exposed place it must be laid in a hidden place, and if it was uncovered, it must be covered.
R. Joshua said: If it is lying in a hidden place, one may lay it in an exposed place, and if it is covered it may be uncovered!
R. Eliezer claiming that it should be treated as potentially clean and therefore not allow any other opportunity for certain defilement. R. Joshua argues the opposite – treat it as already impure, so it doesn’t matter what happens to it.
Why do we care? It is the issue of how we think of things when we have a doubt. Is the benefit given or not?