There are three aspects to editing the field of a cellular automaton. The most obvious consists of inserting, removing, and modifying the cells; actually the number of cells has to remain the same, so that these activities refer to live cells in the context of a binary automaton.
As to the second aspect, for a fixed rule, editing will most likely be used to obtain configurations following a desired course of evolution. This makes it convenient to be able to save and restore configurations, as well as to conduct trial evolutions, all in addition to setting up the configurations themselves.
Finally, although it might seem to be strictly a question of editing the rule table, one often wishes to create a rule causing a given course of evolution. This involves an interaction between observing trial evolution in the field of the automaton and the definition of the transitions in the table. A display of the full rule table is not necessarily required, but there must be sufficient access to be able to modify it.