10 Inch Tonearms

I would use one of the 5-6 tonearms I already own that are not already in use. Or if I needed extra length I’d buy a good one used, but only if the TT required the longer tonearm, not merely to enhance performance.

Since a mono signal is entirely encoded in lateral motion of the stylus, one would think that maybe lateral compliance of the cartridge (not vertical) and inertial mass in the horizontal plane of the tonearm might be critical factors. Azimuth setting would just be for perpendicular physical orientation of the stylus; there is no such thing as crosstalk. Anti-skate is also less critical.

Goofy, the whole issue of tonearm effective length is less critical, if you are going to mount a mono cartridge (not totally irrelevant but much less relevant).  So use whatever length best meets the mounting requirements of your turntable.

Thanks.  I should have also asked whether both tonearms are on the same TT, because the adjectives "faster and snappier" suggest a possible speed difference, if two different TTs are used.

Noromance, same cartridge on both of your tonearms?

Goofy, exactly what are your remaining questions about 10” tonearms?

You actually asked why 10 inch arms exist. Of course, it is an option, because they do exist.

Viv also make 9” and longer versions of their tonearm, all the way to 14”. Viv is an underhung tonearm, entirely different from Kuzma or nearly all others, that overhang the spindle. Underhung tonearms generate much more tracking angle error than overhung tonearms, which error is directly proportional to length. I think Nandric nailed it; some TTs simply do not fit 9” tonearms, so there is a market for 10 and 10.5 tonearms for those TTs. And I do NOT agree that 12” tonearms are inherently superior to 9” tonearms “in every way”. Thus some choose 10 or 10.5 as a compromise .
EDIT.  I misspoke. Of course TAE for overhung tonearms is also proportional to length, which accounts for the existence of 12" tonearms.  What I was thinking is that the very large amounts of TAE that result if you use a 7-inch underhung tonearm like the Viv are drastically reduced if you use the 14-inch version. If you're obsessed with minimizing TAE across the surface of an LP, then you might opt for a 12-inch overhung tonearm (with the attendant increase in effective mass and greater propensity for resonance), or a linear tracker.