Repeatedly one wants to do things like create symbolic links and then call a helper script - the 'interpreter' tag doesn't allow shell commands before calling a helper script so they could not be used in this fashion. The previous pattern for doing this was then to use the ToolShed only 'set_environment' requirement tag. These were onerous to setup and the resulting tools were no longer portable to non-ToolShed installed contexts - I believe this variant is more robust and elegant.
More information https://trello.com/c/0pgF5PBQ and here https://trello.com/c/XK5SqE1i.
Nicola Soranzo