There are many reasons why you might want to gather basic statistics about the data that your devices are producing, especially if you are paying for that data because it's e.g. a cellular link. Is the traffic growing? Are some devices sending more than others?
On the Cohort page, set the Metric to:
Count of $id
This means "count every message" (since the one property that we know every message will contain is the unique device $id). Now to see how the total number of messages from all your devices changes over time, set:
Group by time 1 day
Choose the time period at the top, e.g. 30 days, and hit Run KPI. You'll see a chart showing how your total device message volume has changed over that time period.
To see whether some devices are producing more data than others, remove the Group by time and instead:
Group by property $id
Hit Run KPI. Use the Sort By buttons at the top left of the chart so that it is sorted by Metric, and you will see at a glance whether some devices are producing a lot more or a lot less data than average.
Note: When using Group by property $id, if your device estate is greater than e.g a few thousand devices then you must filter the analysis using the Scope Filter, to limit the number of results.
To refine your analysis, use the Scope Filter at the bottom, creating a filter which matches only certain types of device, or certain types of message.