Hypo is used in photography because of its

.Hypo is used in photography because it is a strong complexing agent.
Sodium thiosulfate is used as a fixer in photography. The negative plate or film obtained at the end of photographic development with a reducing agent (i.e., potassium ferrous oxalate, pyrogallol) contains AgBr which has not been light-activated and reduced to Ag metal. This is treated with sodium thiosulfate solution so that AgBr is removed as a soluble complex compound