That is the question Brittany, at Equipping Godly Women, answers with six reasons why:

  1. The Bible Commands Us to Do it
  2. We Benefit from Living in Community
  3. We Are Called to Serve
  4. Regular Attendance Keeps Us Accountable
  5. We are Less Distracted
  6. It Clearly Demonstrates Our Priorities

Read the full article for details behind Brittany’s list.

While these are Brittany’s reasons, and they are good reasons, we can probably come up with other reasons too. For example, in Manual for Spiritual Warefare by Paul Thigpen, there is a a quote from St. John Chrysostom that says:

When the Devil sees us with others and banded together, he’s not as confident of himself, and he makes no attack. For this reason we have the greatest need to be flocking together continually, so that we won’t be open to the Devils’ attacks.