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It's meant to help give slightly smoother low speed movements, and prevent missed steps. The drivers don't give perfectly smooth sine waves when running slow, until this mod is done.

From the datasheet, the drivers can suffer from "Missed steps in low-speed microstepping", unless auto mixed stepping is used.
The mod gives "Continuous stepping using automatically-selected mixed stepping (ROSC pin grounded)"

A sine wave is shown here, with both those situations with/without Rosc set to automatic mixed stepping

Configuring Rosc is done by either having at 10K ohm resistor (from Gnd) or shorting to Gnd.

The datasheet has a little info about it: https://www.allegromicro.com/en/Products/Motor-Drivers/Brush-DC-Motor-Drivers/A4988