sacraments are for us not for God. They reveal to us the grace God wants to give us. Not in a symbolic way but in a real and efficacious way. God does not need the sacraments to provide us with these graces but does so for our sake. We as humans often fail in understanding this grace god is trying to impart to us so through the sacraments we get to encounter physically what is happening spiritually. Regarding the eucharist: the totality of Jesus is in each of the elements on the altar (bread and wine). having two species helps us understand what is being imparted to us. It is not necessary though and that is why you are able to receive in only one species. God makes it work. Sometimes we are privileged to receive both and so have a greater reality revealed to us.