FortiGuard Web Filtering Service is well-suited for an organisation that is able to funnel all its Internet connectivity through one device, i.e., the office environment - or perhaps where WFH workers are using a remote desktop or VPN. It allows management/IT to minimise risk by blocking websites that may be harmful, as well as websites that may enable data leakage, or even are websites that people ought not to be visiting in the workplace.
If you already have Webroot AV, it makes sense to continue with the same vendor. It's pretty cheap compared to some solutions, so it might be the case that it's this or nothing, in which case this is a no brainer. If you are an MSP, this is an easy product to sell, and requires very little maintenance. you do need to monitor it but it runs itself most of the time. If you are a customer with >50 users, you should really be looking for a product that is a bit higher up the food chain
Enforcing organisational policies regarding internet usage, such as blocking access to certain types of websites (e.g., gambling, social networking, etc.)
Some websites are classified as Meaningless content, especially links used in emails to unsubscribe from lists. Luckily FortiGuard Web Filtering Service is responsive about reclassifying those quickly.
We selected FortiGuard Web Filtering because it came in the service bundle with our FortiGate unit. After testing the service for a couple of months, we disabled the web filtering that was running on our client machines and have not looked back. It's been easy to manage, effective, and fast. There isn't much more you can ask for from a filtering solution.