Simply put, it's a server command that cannot be obtained permanently, via gift from staff or Enjin.
This will hopefully be changed/addressed in the future.
Either /clearinventory /trash or /disposal commands certainly are a popular option as even when using the "/sell inventory" command, there are many items that cannot be sold to the server, which is stuck clogging up player inventories.
It's not about anything other than ease of game function. It allows us to grind faster, and keep it movin'
As of now, /disposal can only be obtained (as far as I know) from Command crates, which even if a player is lucky enough to receive a command crate key, then they only have a 9% (?) chance of getting the command that they want (don't even get me started on higher ranks getting "rewarded" with commands we already have lol..) and if by some miracle we do manage to get it, eventually, it will expire... (5 days)
I'm sure there is more than just one command that cannot be obtained.. But my opinion which is shared by at least a few others is it should be an option.
EDIT: Edited in order to reflect proper use of EULA, Mojang