🚀 go-pugleaf

I have about a dozen small computers in my labgroup, which we leave on 24 hours
a day. We turn the monitors down when we're not using them, but otherwise leave
everything on. We also blow the dust from the insides every year or so.
Having done this for more than a decade, I find that the reliability is quite
high, except for the fans in the power supplies (which tend to fail after three
or four years), and the hard disk drives.

The hard disks tend to last about three years before they start to go bad.
Historically, we've just tossed them out and replaced them. We've not bothered
with doing a low-level format or otherwise trying to restore them to health,
the rationale being that the replacement costs are low and extracting another
six months or so of life isn't worth the bother. 

I'd like to know if this is a sensible policy? What actually goes bad in the
drive? If the bearings and mechanical parts are actually good for many years,
then perhaps we're being wasteful by throwing them out and, instead, 
should just reformat them every few years and keep on using them? I've seen
software adverstised that claims that we should do this, but I've no idea how
good it really works?  I'd sure appreciate any knowledgable advice. Thanks.
Jim