Forum WAS important. Many often i go to forums in different hosting providers and they are (almost) dead when years ago they very active.
Forums are no longer important as use to be and it's not only in hosting related. I think it's a little bit everywhere.
I think users are now using FB Groups.
If you are having more clients and you want to allow them get supported on forum threads then it is a great way to go with.
Beside forum support, you can use ticket or live chat support, it is official way to help your clients.
I was once in a forum that was managed by a hosting company. When I joined the forum, I did not know it was operated by a hosting provider. I was looking for some answers and found the forum. While the forum was very resourceful in providing answers related to the internet technology, I did not think it was helping the admin. The admin was unable to drive sales, thus, he shut the forum.