Nube question - what does a tonearm upgrade do

Just outstanding information ! Far more to this tone arm thing than I had ever imagined . Thank you Larryi & Dougdeacon .
I'm curious...how would the Pro-ject 9 tone arm rate that came on my Music Hall 5.1 table ? What would be a reasonable tone arm upgrade while keeping the stock Magic 3 cartridge ?
Once again...great tutorial !

The biggest improvement rendered to any system I've owned over many, many years of addiction was when I added the Naim ARO to a Linn LP-12. This is the one that I will remember forever.

What makes the difference between a good one and a great one ?

To answer that question correctly, you have to know what a good Arm is, what it cannot do and why a great one is a great one.
Most think, their Arm is a great one (no matter how bad it really is or what flaws it has)... well, what's the difference?
A great tonearm is dynamically balanced, will show rigid construction, fast transmission of energy, rigid bearings which are able to further transport the energy away from the arm, precise geometry, tooling and knowledge of materials.