Why flomax (tamsulosin) is used in kidney stones patients?

Smooth muscle relax: Tamsulosin ("flomax") is an alpha-blocker/smooth muscle relaxant used to open the prostatic urethra/bladder neck to facilitate voiding. Since there is similar muscle in the ureteral wall, "relaxation" should open the ureteral lumen and increase the spontaneous passage of small stones.

Ureter: Because it causes the smooth muscle of the ureter to relax,making it easier for stone to pass if it is not too big.