generally there are 2 kinds of perks that are possible with a TA
1. is if the agency the TA works for has negotiated 'group pricing' or specials which could be for a specific cruise or cruise line and usually are only good for certain time periods or till they sell out. some times they may have some perks that they always offer on all cruises (usually OBC)
2. is agent specific perks - this could really be anything (OBC, special dinning, wine, et all) ultimately a portion of this comes from the agents commission and is often split with the company they work for.
At least that is my experience after 25+ years cruising and 3 years where I supplemented my income working as a cruise agent for a travel agency specializing in cruises.