An Arduino is basically a fancy microcontroller breakout board. The main thing they’re good at that Pis are not is realtime control. The most usual application Arduinos have in 3D printing is acting as a controller for a homebrew printer. If you already have a printer, you don’t need that.
It would be possible to use an Arduino (ideally a Nano or some other reduced-size type) as a separate controller built into a complex homebrew Snapmaker head, but the only head I can think of that would be complicated enough to require it would be the inkjet-type plastic-dying setup someone suggested a while back. Or . . . hmm, maybe a sewing head, with the Arduino controlling the stitches?
One interesting possible auxiliary application for an Arduino would be to use it as the controller for a 3D scanner. It has enough smarts to drive a sensor or camera and a couple of motors to alter the position of the sensor relative to the object being scanned. There are a few project descriptions knocking around the Internet, but I haven’t examined any of them closely enough to see if they contain enough information to build a workable device or not.