Catholics consider Christ's sacrifice on Golgotha to extend throughout all time. It happened once in history, so it can't be a "re-sacrifice", but because Christ is eternal, so is the sacrifice. In other words, at Mass, the celebrants are in a sense present in the upper room as well as in their churches in current time. And yes, Catholics consider the bread and wine to actually be Christ's body and blood, extended and multiplied just as were the "loaves and fishes".
~ and why do Catholics do that - celebrate Mass/Eucharist :
Because at the Last Supper, Jesus said: This IS MY BODY ~~~ This IS MY BLOOD ~~~ DO THIS to Re - Present Me.
That is how and why Catholics believe this.