Why short the pins of the XLR inputs of a power amp?

Hi Gents

My Mark Levinson No 23.5 is magic.

I have just one question: The manual says that while I'm using the rca inputs I have to use a shorting strap between pins 1 and 3. But it does not say why? What difference does it make, sonically or electronically?

I tried a pair of XLR cables between the Primare Pre and the ML but I was disappointed as the sound was not as good as with the rca connections.

Apologies for the daft question but I'm new to XLR. Reading up I see they're for long distance connecting, professional use or for phasing.

I tried googling why shorting is necessary but can't source any useful info.

Thanks!