In most, if not all, other modelling environments, you as the user are stuck with the input and output (display) tools provided by the software developers. This may not matter much if you are working in a specialised area such as electronics. It matters quite a lot for a generic modelling environment, when the range of systems that models can be developed for is so vast, and users requirements for visualising the behaviour of the model are so varied.
All the input/output tools supplied with Simile are Tcl/Tk programs stored as text files in one subdirectory in the main Simile directory. Simile builds up a list of the available tools each time you start it up. Any user of Simile, with access to someone who is prepared to program in Tcl/Tk, can add their own files to this subdirectory, either by copying and modifying the existing ones, or writing new ones from scratch. You can thus provide displays specific to the type of system you are modelling and your own display needs. Having done this, you can then share them with the Simile community. Thus, the whole community can contribute to Simile development!