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.)