Simply put ... to worship God!
Quote from the author.
This is pretty much the reason I go too, and I'd submit, for anyone to go.
Mass, along with other good works of the Church, are viewed by some as "requirements" without a reason, a good reason (like the one above), as if they are a checklist of some sort to get into heaven. Even some Catholics sadly believe this. But even this "checklist" rationale is not a good enough reason to go, after all there's always Confession if one misses a checkmark now and then.
There must be a reason *now* that benefits *us* now, or else even the most die hard rules follower will stumble now and then, and for no reason other than they are a flawed human. So this (the quote above) is the claim all must consider, and all must *expect* with true adherence to the Sunday obligation.
That is, it's quite simple, the Christian claim: Jesus is alive, today, and He wants to help you live a truly human life, now, in every way possible, sparing nothing, denying nothing. The only church to not only offer me this but actually give me this is the Catholic Church, in my experience. As I've said before, I don't know how other "churches" can fulfill such a promise, since the presence of Christ in other churches is at best, a broken presence, only recently "rediscovered" at some point in history where a direct break from the Church was observed, if not an indirect (break from a denomination that broke away from the Church).
But, who am I to argue with another's experience? I can't. If someone reading this who is not a Catholic can honestly say their life has been just as transformed as the authors' and/or mine, and credits this transformation not to their own effort but can honestly say the transformation can only have come from God, then more power to them. I would respectfully submit however, this is not the case, as it's all too easy to convince oneself of anything spiritual; only through careful, rational, reasoned examination of oneself and one's circumstances, can one know for certain a change was due to Christ, and not due to a self-imposed fantasy. And I don't say that to be a wet blanket on anyone's "faith"; merely as a word of both caution and celebration.
Caution, borne of a concern for my fellow man, so that he may not waste his time on this earth chasing after fantasies and empty promises. Celebration, borne of a knowledge that no matter how much time may be wasted, there's always an opportunity to "return home", and come to know the fruits of the "100 fold blessing" right now, here on earth, as the author and I both testify.
Don't believe me? "Come and see" (cf John 1). This is often overlooked I fear in debates about Catholicism. The simplest claim about Christianity is paradoxically the most powerful. And indeed, because we are human beings, it's the only thing that is going to really convince anyone: one's OWN, personal experience. Not an experience shadowed by sarcasm, devoid of reason, or perhaps worst of all, tainted by the evil acts of a few in the Church, but an objectively true experience, in every sense of the word "objective". (and yes, even personal experiences can be "objective").
Jesus, being the great Teacher He is, knew this, knew humanity's need for personal experience so completely and without hesitation, that every act he took interacting with humanity was meant to show who He was (and is) through their personal experience 2,000 years ago (and ours today). Consider this for a moment: He, being God, could have just flipped a switch of some sort and declared, "Everyone is saved, goodbye I'm going back to Heaven now". And that would be it. But He didn't.
Ask yourself "why", and then re-read John 1 again, also asking, "Does this phrase, 'come and see' mean something more? Does it only mean what it plainly reads, a simple request from Jesus to a few Apostles 2,000 years ago, or does it also have a deeper meaning about my relationship with Jesus today?"