we usually use だって as coz, or also(even). eg) なんで寄り道して食べ物を買って食べたの！だってお腹がすいてたんだもん！("Why would you drop by somewhere and get food and eat it?"-" coz I was hungry!") 私の妹だってピアノを弾けるよ　(even my sister can play the piano or my sister also can play the piano.)