What's the benefit of balanced tonearm cables?

I can only answer your question in theory, since it would take actual listening to tell if there is an audible difference in your system between a balanced and unbalanced cable.

Balanced cables are normally recommended for situations where you need to carry a weak signal over a substantial distance. Here are two examples where a balanced cable is probably desirable:
1. carrying a relatively low-voltage audio signal from a microphone to a recording device that is 30' or more away;
2. sending a signal from a preamp to a power amp that is 15' or more from the preamp.

If the distance from your tonearm to your phono stage / preamp input is less than 5-6 feet, it is unlikely that you really NEED a balanced cable. Infact, some audiophiles (including members of this forum) state they prefer the audio quality of unbalanced RCA cables to balanced XLR cables. To determine for yourself, you will have to do some extended comparisons.

A phono cartridge is by nature of the design a balanced source to begin with. If you're lucky enough to have a phono stage with balanced inputs then by all means do take advantage! The benefit over unbalanced is reduction in hum & noise pickup via significantly better common mode rejection ratio.

Or pseudo balanced rca as apparently my arm cables are. I have no idea what anyone is saying! I asked a techie and he said a cartridge can only be left and right so not balanced and the arm guy responded and said my arms are pseudo balanced, so pretty well nearly fully balanced. As I have no way of comparing or altering it is yet another one of those wierd techie issues that drive us normal people to distraction if we stop long enough to wonder what they are all saying.