This is a very significant question. It is also a question that I believe does not get asked enough today. My guess is that if you asked most Christians why the church meets, they would answer something like "to worship God." While that sounds good, the bible tells us that all of life is worship (see Romans 12:1-2).
If the answer is not "to worship God," then why does the church gather? The bible tells us that the purpose is mutual edification (I Cor. 14:26). When the body meets, the point to it all is the mutual growth of that body toward both Christ and Christlikeness.
Some believers suggest that the gathering is for the going. Specifically, they believe that the purpose of the edification is to equip the body to fulfill the Great Commission. But is this accurate? Does the building up happen primarily so that the body will then go out and tell others about Christ?
My answer: body edification happens primarily for four reasons, not primarily for one. In other words, the building up is for equipping the saints for the Great Commission task, but not primarily for that. Edification has four primary prongs, not just one.
When I look in scripture, I see mutually edifying gatherings doing four things. In no particular order these are:
1. Encouraging believers to live holy lives
2. Encouraging believers to sacrificially serve one another
3. Encouraging believers to care for the poor, sick, and needy both inside and outside the body
4. Encouraging believers to take the gospel to the ends of the earth
As we read through the pages of the book of Acts, we see all four of the above in action. Later, when we read the epistles to the churches, we see exhortations to all of the above. None of the four appears to be demonstrably more important than the others. All are key aspects of the Christian life. We do well as believers to edify one another toward all these things. We also ought to take stock of our own lives to see if we are ignoring any.
So, is the gathering for the going? In part yes, but it is so much more than that. Let's edify each other to holy living, to sacrificially serve others, and to take the gospel where it isn't. Christ both empowers these and expects these from His followers. What great privileges we have in Him!