See image above...2021 Accord 2.0 Touring - what is this bolt for and should I be concerned that the driver side is loose (spins freely) and the passenger side is not? I attempted to tighten, and it just spins like there is a loose bolt on the back side of it...

 

From looking at a strut photo for your Accord it is unlike the other three as it doesn't hold the strut to the body. it is part of the strut itself.

 

I'm not familiar with the 2021 setup exactly. The other 3 brass colored nuts are holding down the upper shock plate. The loose silver nut looks like it is aligning for proper orientation during installation. It doesn't look like it is clamping down on the white subframe.

I suspect the threaded bolt on the other side is knurled like a wheel lug, so you might be able to reach under the wheel well and push up on the back side of the lug/bolt and try to tighten? If you can see in there, you should be able to see if that nut is securing anything. After a second look, does the threaded portion of the loose bolt have an allen key where you can hold the threaded portion?

 

I just realize it is an alignment device for the strut installation but someone beat me to the answer.