Why is mass important to catholics?

Here are just some :)

It is commanded of us by God when He asks us to keep holy the Lord's day.

This is where we receive Jesus in the Eucharist (body, blood, soul, and divinity), and receive graces to preach His Word. The Eucharist is only received at Mass.

It is when we gather as the Universal Church to pray for all in need, and have fellowship with one another.

There are many many reasons. Feel free to ask any parishoner or priest for more elaboration :))