I was just reading this article – On the expression of recentness and the English past progressive

Abstract

Due to its poly-functional nature there has been an unbroken fascination with the English progressive construction and the various meanings associated with it. The motivations of these different meanings and their historical developments have found ample attention in the literature on the English verb phrase. Apparently, there is not just one straightforward form–function mapping for the progressive. It not only functions to express various notions of imperfective aspectuality, which are typically given to be temporariness, i.e. duration and limited duration, as well as incompleteness, etc., but it is also employed as a marker of non-aspectual pragmatic or subjective meanings, as for example in the signaling of politeness and discontent., The progressive in contemporary English owes its functional diversity to a continuous development and evolution, possibly starting out as the conflation of two constructions in Late Middle English, viz. the earlier Old English periphrastic construction consisting of beon/wesan plus an often adjective-like present participle ending in -ende (OE he wæs feohtende), and a Middle English locative prepositional construction of the kind be on/at V-ing (ME he was on huntunge) (see Visser 1972: 1095; Jespersen 1909–49: iv: 169). © Cambridge University Press 2013.