What's the benefit of balanced tonearm cables?

My phone stage (bat vkp10) has xlr and rca inputs. bat vk50se preamp. I use all balanced cables for everything except the tonearm cable.

What's the benefit between your cartrige to phone stage?

From the phono amp designers I have spoken to over the years it's not a simple case of one design being better than the other, but a question of implementation.
I recall one designer saying that he gave a balanced option to satisfy the demands of the high end buyers i nthe states and far east. He said that introducing the extra circuitry for the grounding in a balanced set up caused of itself problems, and wrongly done can cause sound.
Two of the greatest stages of all time were un-balanced - Vendetta and Mares/Connoisseur.
Balanced signals have greatly reduced common mode noise,

Just to be precise, balanced signals can have common mode noise, maybe even a lot. It's that balanced circuits reject the common mode noise and only amplify the differential signal.

How do I convert a single-ended tonearm cable, with the ground wire separate (2 RCAs per channel plus a thin ground cable) to a balanced tonearm cable using two male XLRs? I use an Ortofon Kontrapunkt b MC 'low output' cartridge.

The tonearm cable is ’integral’ with the tonearm, so I can work on the output end of the tonearm cable only.

The phono preamp has a true balanced circuit for MC cartridges using properly wired XLR male connectors from the tonearm cable. The preamp does have RCA inputs for MC (and MM ) cartridges. If I use the RCA inputs then it 'converts' the MC to single ended at the input only. If I use XLRs in a balanced fashion at the MC XLR input then the entire signal chain is balanced. The circuit is truly balanced.  I have seen the block diagram.

Thank you in advance for your advice on sewing this cable.
If you are sure that your phono stage contains a true balanced circuit inside and is not just using XLR input jacks, then please also tell us what kind of interface there exists between the tonearm and the phono cable.  If it is a DIN 5-pin interface, then you would have a female DIN plug at the end of your phono IC.  Inside the tonearm, 2 pins are used for hot and "ground", respectively, of the R channel, and 2 other pins are used for hot and "ground" of the L channel.  The middle or 5th pin is connected to that dangling ground cable to which you referred.  Don't worry about that at all; use it as a ground or don't connect it.  You can go and buy a balanced phono IC with a female 5-pin DIN at one end and an XLR at the other, and you should be good to go. But make sure the manufacturer guarantees the IC is wired for balanced circuits.  What happens is that the R and L "ground" connections from the DIN plug are now used for the negative phase of the balanced signal, while the hot side carries the positive phase of the balanced signal.  Inside the balanced cable, there should be two identical in quality conductors per channel, one for positive and one for negative.  That's key. The pos phase by convention usually goes to pin2 of the XLR, and the neg phase goes to pin3.  Pin1 is reserved for any shield or ground internal to the cable itself. 

In a single-ended cable, the "grounds" side of the R and L channel outputs, respectively and the shield are connected together, at least at one end, and solder to the barrel of the RCA male connector at the output side.  The phono ground conductors in an SE cable need not be of the same quality as that which carries the hot side.

@ivanj you haven’t understood what was written above. Try reading it again. 
ALL cartridges are balanced. The cable type (RCA or XLR) doesn’t make a difference. 
If you have RCA outputs from your tonearm cable but your phono preamp only accepts XLR, just get RCA to XLR adapters, being mindful to keep the ground wire of the tonearm cable connected to the phono preamp.