From the PDF introduction
A Manual of the Planes Web Enhancement
No creatures are more alien and mysterious than the ultralawful modrons. In this web-only supplement, available exclusively from the Wizards of the Coast website, we offer statistics for these geometrical creatures, plus an extensive background on their history, society, life cycle, and relationships with other planar creatures.
What’s a mortal to make of the modrons-those strange creatures of absolute order that whir and click along in the Clockwork Nirvana of Mechanus? To an outsider, they seem to have no existence other than as a whole. Indeed, there is a saying: "To look at one modron is to look at all of them."
It is only logical that these creatures are native to Mechanus. Rumor has it that they are the keepers of their mechanical plane, the maintainers of the gears and the polishers of the cogs. Modrons keep the whole place running smoothly and cleanly-without them, Mechanus would surely break down.
Though the majority of modrons live in Regulus (their own city in Mechanus), they can be encountered anywhere within the planes. No modron is ever without a task to carry out, though these tasks may be no more comprehensible to other creatures than the modrons themselves are.
Just what are the goals of these creatures of ultimate order? Do they want to impose total law over the rest of the cosmos? Are they simply keepers of the machinery that drives the multiverse-the repair unit of infinity? Or are they devious players on the cosmic gameboard, trying to eliminate their competition? These questions may never be answered, and a host more may never be asked. No one but a modron truly understands a modron.