I was our test environment as a spoke network replica. Since most of our spoke locations are pretty cookie-cutter, I can easily scale my test results from one network to dozens. Also any updates that we do that would affect our spoke networks and the equipment (Meraki or not).
From there I test pretty much anything we don't use on a daily basis. VPN Setup, new features, baseline testing, policies, bug bounty hunting :-). It is a test environment so I like to break it because I know I can just rebuild it.
API provisioning, using cellular as WAN 2 interface on the MX, HA/Spare MX configuration/troubleshooting etc. We are testing a lot of new Meraki stuff including the cameras, MX, bringing some cellular stuff into the mix, throughout this we are finding some peculiar problems within the dashboard, such as the primary MX showing as the "Current Master" while the spare device shows status of "Current Master" as well.
We also test out a lot of the SNMP capabilities within the dashboard/devices. To ensure we can monitor/alert on our Meraki devices out in the field.