I can't say why people use the phrase today...perhaps sometimes Perhaps it is sometimes used as a subtlesubtly sarcastic rebuff in sarcasim toward someone who is not demonstrating real mercy or kindnesses...but rather, but a self serving-serving appearance of largesslargesse.
But I believe the phrase in the Shakespeare text I believe is:
...to be a gentle gently imploring for mercy toward the asker...by by reminding the person implored...that that it costs nothing to show mercy...that in fact — that to do so, in fact, blesses all concerned.
The gentle rain'gentle rain' metaphor demonstrates this...the: the Bible says (as Shakespeare well knew) that 'God sends the rain to fall on the just and the unjust'...notunjust'; it is not that heGod is ultimately not also in a position to finally judge...butpass judgment, but that Godhe is merciful...frequently, frequently giving better than we deserve...and, and is quick to forgive those that truelytruly seek suchthe same with a bowed heart.
It 'straineth' not to be merciful...but, but overall makes one richer over all in character. It'sIt costs one nothing to forgive...except ones, except one's own smallnesspettiness. ForgivingTo forgive is not small...norpetty, nor is it to be brushed aside as a casual...but it thing, but is ultimately large in the end.