A personal reason: I'm building a global simulator for shadows [1]. I started with SRTM based elevation data which allowed me to cast mountain shadows. I'm now offering some LiDAR data which also includes buildings, structures and trees (~50cm resolution). People use my website for sun mapping their gardens, real estate, farms, events, photography, academic research, solar systems. It would be nice to amp up that resolution by another factor.

[1] https://shademap.app

Edit: after reading the article it seems less about surveying the Earth's surface and more about accurate triangulation like GPS :(

Making better maps seems like an obvious usecase, and by "maps" I mostly mean technical documentation. I assume most EU countries have a requirement that new building permits need to come with precisely measured outlines, this could help increase their precision and/or make the measurements cheaper.

In case we need to shrink the Earth for Intergalactic travel....Somebody knows something...

Sometimes we need to give awesome tools to creative people and see what they come up with, even when we don't understand the implications ourselves.

I think millimeter accurate GPS is one of those tools. It has the power to enable so many things. Things we cannot imagine without using the tool itself.

40 cm vs 1 mm is the difference between landing a quadcopter smoothly or crashing it into the ground.

20 cm vs 1 mm is the difference between a robot navigating through a door or crashing into the wall.

20 cm vs 1 mm is the difference between mowing the lawn or cutting through your flower bed.

Unfortunately it doesn't look like we'll be getting millimeter accurate GPS anytime soon. The Genesis satellite might be a prerequisite though.

- The satellite will accomplish this [precision] by having the usual main Earth-measuring techniques co-located on board [satelite navigation, interferometry, laser ..] When used together, the ESA expects to be able to correct for biases inherent in each technique.

- An updated International Terrestrial Reference Frame (ITRF) will have immediate benefits on satellite-based systems, impacting Galileo-enabled applications in fields like aviation, traffic management, autonomous vehicles, positioning and navigation

- The space agency added that meteorology, natural hazard prediction, monitoring climate change effects, land management and surveying – as well as the study of gravitational and non-gravitational forces as fields – would also see benefits.

- ...

> Why?

So somebody can tell if the neighbour’s fence infringes on their property line.

Sea level rise.

To make another one to verify if 42 is indeed the answer to everything. ;)