perché = because, why... e.g.: -Perché non sei venuto ieri sera? -Perché ero malato. -Why you didn't come yesterday evening? -Because I was ill. poiché = because, given that, since... e.g.: Poiché sono stanco, vado a dormire. Since I'm tired, I'll go to sleep.

Yes there's a fundamental difference. "perché" exists, "poiché" doesn't.