It creates a regular communication and expectations.
A regular monthly report/phone call gives you a good opportunity to chat about any issues that may have come up over the last month and lets them know you've been busy working hard on their account over the last month.
But a lot of the monthly reports I've seen are pretty plain and just don't represent the image an online marketing business should. That's why I've attached a free template you can rip-off and use for your own clients.
You don't need to use this exact formula for your template. In fact, I encourage you to make it your own. But if you want to save some time and have some professional reporting, I encourage you to have a look.
My other suggestion is that you send your completed client reports in .pdf format.
Let me know if you've found the attached doc helpful or completely useless. Cheers!