i still don't get it, although i should as a lover and user of abstractions: using systems theory, foucault, structuralism and the second-order-thing since 20 years. so could someone explain the relevance of this "form" concept to me again, without the historical context of scientific discourse. at which point do i really need this?

Spencer Brown's notion of form is an idea, which, in a nutshell, puts quite a lot of post-structuralist, deconstructive, and constructivist thinking, reading, and doing of the last decades into a single tool: Any cognitive operation (by cells, organisms, consciousness, or communication, depending on what you are interested in) is a distinction, indicating a marked state, which is considered to be a state produced by the crossing of a line between that marked state and an unmarked state. An observer, doing its own cognitive operations, is able to look at the 'form' of that distinction, distinguishing (a) a space any distinction is producing by assuming it, (b) the marked state indicated, (c) the dividing line produced by the very distinction being produced, and (d) the unmarked state running with the indication of the marked state as the outside of the distinction unkown to the first-order observer, but to be indicated, at the latter's risk, by the second-order observer.
You need that concept only if you are willing to enter a radical ecological thinking, doing away with any ecosystem, having to deal only with ecocomplexes, where neighbours watch, and ignore, neighbours.