Eridium comes in purple bricks in Borderlands (I'm only talking about Borderlands 2 here, but I'll leave out the 2 to avoid going nuts) that you use to upgrade your character at Crazy Earl's in Sanctuary.
When I first started playing--years ago--I figured out that you could use the slot machine's at Moxxi's to build up Eridium. If you had the cash, you could build up a big stash and upgrade your character attributes. For low level characters, those upgrades are crucial: backpack slots, ammo limits, grenade count, etc. Of course, building up the cash to build up one's Eridium is difficult in normal game play.
The "Assault on Dragon Keep" DLC or what is sometimes called The Unassuming Docks (of potentially little importance) changed all that. That DLC is designed to be done after the main story. As such, the minimum recommended character level is 30. Even at 30, it can be a tough slog to get through the Unassuming Docks to Flamerock Refuge. I used to not care so much, but it turns out that Flamerock Refuge is crucial because of the Tiny Tina slot machines in that Ren-Fair incarnation of Moxxi's (which doesn't exist until a Torgue mission summons the tavern into existence). Tiny Tina's slot machines in that tavern are the crux of what I alluded to in my title.
The thing about Tiny Tina's slot machines is that they run on Eridium, 2 per pull. But, crucially, they have a frequent 3-rainbow payoff of cash, lots of cash. The payoffs of cash are so over-sized that it creates a loophole in Borderlands economics. You can spend all your money in Sanctuary's slots to get Eridium to spend at Flamerock Refuge's slots, but in the process, you will more than win back whatever you spent to get that Eridium. This means you can, ad infinitum, go to Sanctuary, build up Eridium, and then go to Flamerock Refuge to spend it, simultaneously recovering all your cash in the process.
Why? I mean one slot machine is as good as another, right? No. Not at all. Tiny Tina's slot machines in Flamerock Refuge not only give out great weapons, shields, and grenade mods, but also legendary and teal weapons. The legendary part is true of the Sanctuary slots as well. Get the vault symbols in Sanctuary and you get a legendary weapon. But in Flamerock, if you get the vault symbols on the slot machine, you get a teal weapon (sometimes called pearlescent). These are usually the best weapons in the game, often with attributes that you just don't get anywhere else. For instance, some teal weapons will automatically regenerate ammunition.
Also at Flamerock, there is a 'spinny ball' thing wherein if the machine comes up three dice (literally 20-sided D&D dice....the whole DLC is a D&D campaign), the drawer opens and a die spins in the air before dropping onto the drawer. The 20 is replaced with a vault symbol. If you roll that vault symbol, you get a legendary weapon. But most important of all is that if you roll a 18 or 19, you get three Seraph crystals. This is the only place in the entire game where you can quickly and reliably farm Seraph crystals.
Seraph crystals are the only things that Seraph Vendors will take. They are the only way to buy an Antagonist shield as well as some other remarkable gear. If you haven't found any of the Seraph vendors, google it. There's one in each DLC. Each specializes in different kinds of gear. The one in Flamerock sells Antagonist shields.
Do you start to see what I'm getting at now? Access to Flamerock refuge gets you the ability to infinitely farm Eridium to get legendaries, teals/pearlescents, and Seraph Crystals.
I consider this Sanctuary/Flamerock slot-machine arbitrage crucial to building a strong character. So crucial, in fact, that I will go to the Unassuming Docks and rush the gate to Flamerock Refuge even though I have no hope of fighting anyone in the docks. All I need to do is to get Bony Pants Guy to spawn, opening the town gate so I can run through to the Flamerock gate. Once you get there, Flamerock Refuge will appear on the fast-travel menu for the rest of the game. You can come and go as you please.
That's not to mention that the Assault on Dragon Keep is the most entertaining DLC, and loot-riffic. Once you complete the story missions, you gain access to Murderlin's Temple which...is a discussion for another post.