TTL simply means that the camera tells the flash to send out a "preflash" burst of light at a known brightness level that the camera can meter, so it can have a sense of how to set the flash power for the actual shot. Distance, focal point—it still just gets metered. Where the flash is doesn't really matter—the metering itself is still done from the camera.

However. TTL triggers aren't necessarily prized by off-camera flash shooters for supporting TTL function. TTL triggers communicate most of the full hotshoe electronic communication protocol—not just the sync signal, like manual triggers do. So, more of the on-camera function of an OEM flash can be used with a remote flash—like controlling the flash mode, or the manual power level or FEC adjustment with TTL remotely. HSS/FP can also be used. 2nd-curtain can be used. Adjusting settings on the flash remotely. These can be extraordinarily convenient, vs. having to walk up to each individual flash to adjust settings.

So, no, a TTL-capable radio flash trigger is not meaningless.

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[addendum written 2021].

These days, TTL triggering is even more meaningful, because in 2015, Profoto added a feature to its Air triggering system: TTL Locking. And everyone rushed to copy it, so that by 2018, even Godox and Cactus had it, and today most TTL/HSS monolight system triggers and OEM radio flashes have a similar feature (with Canon's ST-E3-RT Vers 2 probably being the last to follow suit in 2021). But because it's such a new feature, many people don't know about it, let alone are proficient with it or teach it. It also doesn't help that, as with tail-syncing, everybody has a brand-specific name for it:

• Profoto doesn't have a name for it, just "switch to Manual"
• Godox: TCM (TTL Convert to Manual)
• Westcott/Jinbei: Equivalent Manual Exposure
• Cactus: Flash Power Lock
• Nissin: TTL to Manual Conversion
• Canon: FE Memory
• Sony: Memory Level

The main complaint against using TTL for off-camera flash is that because it's metering-based adjustment, shots vary their flash exposure shot-to-shot as the composition changes. And this was true. But with TTL locking, you can use TTL instead of a light meter for initial power-setting, and then lock that TTL-set power level in as an M setting. You can adjust before locking with TTL FEC; or after locking with M adjustments. And what most people don't realize is that TTL is likely to get the right power level on the first shot without adjustments. It surprised me how well it worked when I finally got around to trying it.

Most folks who teach lighting with off-camera flash learned back in the days of film, or before TTL radio triggering existed. So, they learned to create a fast, efficient manual workflow. And that's what they teach. But they inadvertently lock down iso, aperture, and light placement early in a shoot because of the hassles of adjusting the power settings on the lights to match any change to those three factors.

But TTL makes all three of those factors transparent to flash exposure (within gear limits). So you don't have to lock yourself in early. You can continue to refine, dink, adjust, and dynamically flow from one setup to the next if that's how you like to work. When you need shot-to-shot consistency, lock the settings down. When you don't, go back to TTL.

The only real drawback is that getting ratio control between groups is impossible if you mix different sized lights (say speedlights and monolights). And all the 3rd-party radio triggers only do group FEC for ratio control, so they're limited to (typically) ±3EV FEC compensation ranges, which means you can't go past 8:1 to 1:8 ratios. And they do not do TTL group ratios like OEM flash triggering systems can (examples: Canon, Nikon, Fuji) when all the groups are in TTL. So, TTL may only be useful for your key light or in a one- or two-light setup. There are still reasons to prefer M. :)

But TTL for off-camera flash can let you do things like switch from f/5.6 to f/1.4 on the fly and back again, without having to break your concentration or connection with your subject to adjust the lights manually, as Joe McNally demonstrates in this Profoto promotional video.