Some older (and stereo) receivers don't have a LFE/line out for the subwoofer, just speaker wire, so routing into the sub with wire (and out again to speakers) is one way around that. It's not the norm for most setups nowadays, but manufacturers often build things with flexibility in mind (hence receivers having a bazillion inputs, speaker inputs on subs, etc.)
If you get a new receiver with a line out, you can ignore the speaker wire inputs.
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