What does a tonearm contribute to the sound of a turntable?


Curious about how a tonearm affects a turntable sound. I guess it's the piece of the turntable I know the least about and feel the least connection with. how does a really good tonearm affect the sound or not affect the sound? And what about the tonearm does the affecting?
128x128simao
If the Jelco dropped the noise floor, you should most likely have to turn the volume down, not up.
An interesting update:

I just got in and installed a set of Morrow Ph-4 phono cables, DIN-to-RCA. They replaced the ancient Sumiko PIB-1 junction box fro which I was running Acoustic Zen Silver Reference 2 IC's to the phono pre.


Morrow's literature advised a millenia-long break-in time, like 500 hours for the cables, so I swapped out my relatively new Hana SL cartridge for my old Benz Micro Ace S cartridge so the latter could take the brunt of the break in time and wear and tear. I Baerwald aligned it, adjusted the tracking force, and sat back for a few listens.
Holy crap! Just a bigger, more full sound than the Hana ever produced. Maybe it's the Benz's 8.8 gram weight versus the Hana's 5 gram that made the former play better with the new Jelco 750 in terms of compliance. Idk; but there was a palpable presence that hadn't been there before.


I know - lots of changes in variables here: new phono cables; new DIN connection; new cartridge.

Thoughts?

Not to mention I had to adjust the VTA after reinstalling the arm, so that may be a factor as well.