What is in it
Group activity: how many messages, how many people sent them, how the week compares with the one before, and who the most active members were. It is computed from the messages the bot stored, so it describes your group rather than estimating it.
On demand or on a schedule
Pull it when you want it, or have the bot post it into the group on a schedule. The second option is the one worth trying: a visible weekly summary tells the group it is being looked after, and it quietly rewards the people who show up.
Reading it in thirty seconds
Look at active senders first and messages second. Senders rising with flat messages means more people are joining the conversation, which is good. Messages rising with flat senders usually means a few people are having a long argument, which is worth reading before it empties the room.
The fixed format, and why that is fine
There are no custom templates. In exchange there is nothing to configure, nothing to break, and the report looks the same every week, which is what makes a trend visible at all. If you need a different cut, the CSV and JSON exports are there for a spreadsheet.
What it cannot tell you
It covers the period the bot has stored, so it starts when you turn the module on and reaches back as far as the retention window. It also cannot see what happened in private chats or what people meant. A report measures traffic; reading the room is still your job.