The Basic Programming cart for the 2600 was made mostly to satisfy an advertising promise. Atari had advertised the console as expandable into a programmable computer, but there wasn't any way for a user to do it. There were threats of a class-action lawsuit, so Atari made that cart to head that off. (I remember this from years ago, though can't find a cite for it now, the search terms are too vague and wikipedia doesn't mention it.) It wasn't really intended to be a usable programming environment for anything useful.