If testicle torsion is left to long why does it have to be removed or can you not remove it and what are complications if not removed?

If not removed gangrene , an infection, can set in and be life threatening. Note after more than 12 hours of torsion the testicle is very likely infarcted and not functioning ( more than 75%) and it is unlikely that function can ever be restored. Cosmetically you can get a prosthesis. https://my.clevelandclinic.org/health/diseases/15382-testicular-torsion

Torsion left too long causes loss of blood supply and tissue death(necrosis) This can either cause atrophy ( death an dshrinkage) of the testis or sometimes causes gangrene which is a type of infection and is serious if not removed. To avoid that chance the testicle is removed