Calipers should be rebuilt when the pistons appear to be sticking, there is leakage, subject to excessive heat such as a fire, the brake fluid is dirty, or it has been several years since the last rebuild.
Rebuilds are relatively simple. Often it's just an o-ring kit and cleaning. The pistons are stainless steel which are normally fine. Since the caliper bores are aluminum, it's best to use cleaners and to avoid anything abrasive unless necessary. You can use compressed air to coax the pistons from their bores. Lubricate the bores with brake assembly fluid or brake fluid, lubricate the o-rings & pistons and reinstall. Be sure to bleed the brakes and check for leaks after installation.