Actually, the doubled strings of the mandolin family are to assist in tremolo playing. When the pick moves across the first string in the pair, it starts it ringing. When it crosses the second, it rings. Then, when the pick comes back -- remember, the technique is to play very fast up-down-up-down strokes, attempting to produce a continuous tone -- it momentarily interrupts the ringing string as it picks it, BUT THE OTHER STRING IS STILL RINGING.

That means that at least one string is still producing the note while the other one is being struck by the pick. By the time the pick, now travelling upward, reaches that (still-sounding) first string, the second string has been picked again and is now sounding. A more-continuous sound is the result.

Think "riiiiiiiiiing" instead of "r-i-i-i-i-i-i-i-n-g." Try the technique on the single strings of a guitar and you'll see what I mean.

The added volume was just a by-product.

Bob