Thanks for Bob's answer. To make it stronger, now I learned that the necessity of fork as below.
The motivation of introducing fork is to implement tacit programming as a realization of Combinatory logic. you need sort of basis combinators for this ( like the s-k basis on wiki) , and hook/fork form a complete basis. Fork or its equivalent is really unavoidable for this purpose.
the concept of fork is natural if thinking of
f + g
,f * g
. In math they usually meanf(x) + g(x)
andf(x) * g(x)
.This topic was explained nicely in Roger Hui's essay on verb trains.