1. If we know what the neurons do, we can start trying to understand how they do it. Like at the circuit level. And if we can figure that out, this will very likely help us figure out what other neurons are doing and how their circuits work. Currently very little is understood.
2. There's a lot of speculation that grid cells are also used for higher-level cognition, to represent relations between more abstract things. This is one of the two main theses of '1000 Brains' by neuroscientist Jeff Hawkins.
Why are these two things interesting? Well wouldn't it be cool to understand how the brain works? It might help us build AI that learns as fast as we do (chat GPT was trained on more text than one could read in many lifetimes). It might help us augment our own intelligence. These are very long-term goals of course, but understanding grid and place cells seems like a reasonable starting point.
Another application is in computing. Advances in the understanding of neuroscience have stimulated the creation of artificial neural networks (for example, the convolutional neural network was inspired by the cat visual cortex). Understanding how the human brain encodes concepts such as time or space might help us to design artificial systems.
Finally, the most important aspect of this work is (imo) advancing basic science. We don't understand how the brain works, and work such as this brings us closer to solving the grand mystery of neuroscience.
and thanks for reminding me! i've been meaning to pick katalin kariko's book back up!