I hang out at AudioKarma, and paid subscribers get a badge under their username, plus access to a subscribers area called Barter Town. I believe they get a couple of other perks also.
On one big board I run, we have a Marketplace area where members need to be signed up for at least 90 days, and have at least 50 posts, before they can even see the Marketplace area. We had problems in the past with members joining only to peddle their junk. We even still get a few, but the community aspect of the site tends to drive them away if they are peddling junk.
That is one area where having a paid subscription is nice--you can lock certain areas, making them available only to certain members who are serious about the forum, and can be more trustworthy in buying/selling items to fellow members. This is the idea behind what AudioKarma does on their site.
I had considered it to help raise money for my sites, but I'm having good enough success with Viglink that after I rejuvenate my sites and the money rolls in, I will be covered. However, I had considered offering a subscription, where members would get the ability to change their user title, upload attachments, get a few limits raised, and I even toyed with a subscribers-only marketplace. It still may have a spot on my forums in the future, but it is no longer a necessity.