In German language why we put zu and an sometimes in the end of sentence?

Put simply it's just the part of the verb, translated in the same word order as the German you get: "Look the photos at" (Sehen Sie die Fotos an). However, this word order isn't used here in English, we say instead "Look at the photos". Instead try "Zieh'
deinen Mantel an" = "Put your coat on" or "Put on your coat". Without the preposition, the clause makes no sense, either in German or English, but in German the preposition always goes to the end of the clause or sentence .