Actually that is not so much the main point, but rather exogenous spending changes are a mechanic by which Keynesian types seek to increase demand.

Supply-side economics also uses this concept, but in terms of tax cuts rather than spending increases. I consider the mechanic faulty , and attempts to manipulate it as one of several primary causes of malinvestment - that is, misplacing resources.(which is a prime driver of bubble economies... which are incedentally caused by inflationary and stimulative monetary and fiscal policies). I also think that Keynesians have a poor understanding of the effects of interventionist spending in a global economy and also frequently mistake capital flow with wealth creation. But I digress.

But anyhow - govt spending is the mechanism by which Keynesians frequently like to attempt to influence the multiplier, simply because it is an easy lever to pull via centralized control. But the purpose of pulling this lever, is still to address the Keynsian concept of the "demand problem".

For who could be free when every other man's humour might domineer over him? - John Locke (2nd Treatise, sect 57)