For hardcore WoW raiders, level 60, 70, and 80 raids are pretty much obsolete content. You can't get any gear upgrades, and most of the fights are trivial at level 85 especially if you're in full epics. However, there's several good reasons to revisit the raid dungeons of yesteryear, for fun and for profit!
It's All About The Gold (And Loot!)
While Vanilla and Burning Crusade raid bosses have had their amount of gold nerfed substantially in Cataclysm, bosses in Wrath of the Lich King raids such as Naxxramas still drop a whole lot of gold. Since 10-man Naxx can easily be done with a group of 4 or 5, farming the better part of the raid will provide plenty of gold, Abyss Crystals (if you have an enchanter), cloth, and even some valuable vendor trash!
Rack Up Some Achievement Points
If you enjoy completing achievements – both personal and guild achievements – classic raids should definitely be on your to-do list! Completing the Classic Raider, Outland Raider, and Lich King Raider achievements can be both fun and rewarding. Many of the higher tier bosses (such as those in Sunwell and Icecrown) still require a bit of strategy to beat even at the level cap. Learning and practicing these trickier boss fights can be good practice for progressing into current raids. Also, it can help your guild practice working together as a group.
It's More Fun Than Wiping
Running classic raids instead of progression raids can be a great stress reliever, especially for the hardcore raider. It's relaxing to take a night off from progression raiding, and blow off some steam by beating up some old raid bosses. Since everyone won't need to be as focused as in a progression raid, it can provide the perfect environment to socialize and laugh it up with your guild!
Build Your Guild Reputation
When in a guild group, everyone in the group will earn guild reputation, and your guild will earn experience points as well. Farming some old raid dungeons is a fun and easy way to build reputation with your guild, and help your guild level up. If your guild isn't level 25 yet, or you're not exalted with your guild yet, the extra XP/rep will come in handy.
Do It For The Epic (and Legendary) Loot
While the items that drop from old raid bosses may not be as powerful as current tier raid bosses, building a collection of tier sets or other items can be fun to do. It doesn't stop there, though. Legendary weapons can be either built or farmed from many old raids. Illidan, the final boss in Black Temple, has a small chance to drop legendary items such as the Warglaives. Or perhaps you'd like to build a Val'anyr from doing heroic modes in Ulduar. Having a legendary weapon will also give your guild an achievement, too! While these legendary weapons still aren't as powerful as their level 85 counterparts, having one is definitely an accomplishment worth showing off!