Drain it, baby!

I had the shivers this week-end: my HP dv6000-series laptop would refuse to charge the battery. I kept begging it, but to no avail: it gave me the blue blinking light. And this is, how should I put it, bad. It means that the battery needs to be replaced.
For heaven's sake, that battery was less than a year old! This is the 21st century! That stuff was invented in the 19th! And they still can't make those work reliably?
That's when I remembered of some obscure procedure one person at HP tech support told me about 9 months ago: The Power Drain (tm). Yes, believe it or not, there is a way to drain whatever power may be left out of your HP laptop...
The draining goes like this: unplug the AC, remove the battery, and then press the power button for 20s or more (the first time HP made me do this, I thought it was a joke!).
I tried it this time. Then I placed the battery back in its compartment, plugged the AC back in, and lo! the battery started charging again. Everything seems to work fine now.
As to why some drainage was necessary...