Since solder is such a poor conductor, why use it?

Al's first response is excellent, he obviously knows what he's talking about.

It's best for the materials being soldered to already have a good mechanical connection before soldering. The solder just holds it in place.

However, solderless set-screw connections do tend to sound best but the difference vs using good lead-free silver solder like WBT or Johnson is slight. The best RCA plugs imo, WBT 0102, are the best despite requiring soldered connections.

Davec113, yes, solder is good for holding in place, but so is super glue. It is not conductive, however, and may harm the cables. However, the solder even for tightly crimped wires would flow between the wires.

Use a length of solder wire in a circuit and listen to what you've got. Crap!

The best connection is probably wire wrap. With wire wrapping. Some audio gear made in the 1960's and 1970's used this approach, but for cost reasons, it is no longer used. The connection is physically very strong and completely gas tight (no oxidation at the physical point of connection). See the following:

https://en.wikipedia.org/wiki/Wire_wrap

The link Larry provided is a good summary of wire wrap technology. With respect to its (non-)applicability to audio, though, in addition to the cost factor I would highlight the following paragraph:

Wire-wrap works well with digital circuits with few discrete components, but is less convenient for analog systems with many discrete resistors, capacitors or other components (such elements can be soldered to a header and plugged into a wire wrap socket).[2] The sockets are an additional cost compared to directly inserting integrated circuits into a printed circuit board, and add size and mass to a system. Multiple strands of wire may introduce cross-talk between circuits, of little consequence for digital circuits but a limitation for analog systems. The interconnected wires can radiate electromagnetic interference and have less predictable impedance than a printed circuit board. Wire-wrap construction cannot provide the ground planes and power distribution planes possible with multilayer printed circuit boards, increasing the possibility of noise.[3]

For those reasons it is pretty much inconceivable to me that something like a high resolution DAC, a high gain phono stage, or any number of other examples that could be cited, could be implemented successfully with wirewrap construction. And then there are the high power devices in power amplifiers, that require special mounting, heat-sinking, and heavy gauge connections ....

Also notable:

Surface-mount technology has made the technique much less useful than in previous decades.

Regards,
-- Al

Almarg, I'm not recommending wire-wrap only something superior to solder. Mini welding would seem the best solution.