Question: "Why is church attendance / going to church important?"
Answer: The Bible tells us we need to attend church so we can worship God with other believers and be taught His Word for our spiritual growth. The early church "devoted themselves to the apostles' teaching and to fellowship, to the breaking of bread and to prayer" (Acts 2:42). We should follow that example of devotion—and to the same things. Back then, they had no designated church building, but "every day they continued to meet together in the temple courts. They broke bread in their homes and ate together with glad and sincere hearts" (Acts 2:46). Wherever the meeting takes place, believers thrive on fellowship with other believers and the teaching of God's Word.
Church attendance is not just a "good suggestion"; it is God's will for believers. Hebrews 10:25 says we should "not [be] giving up meeting together, as some are in the habit of doing, but encouraging one another—and all the more as you see the Day approaching." Even in the early church, some were falling into the bad habit of not meeting with other believers. The author of Hebrews says that's not the way to go. We need the encouragement that church attendance affords. And the approach of the end times should prompt us to be even more devoted to going to church.
Church is the place where believers can love one another (1 John 4:12), encourage one another (Hebrews 3:13), "spur" one another to love and good works (Hebrews 10:24), serve one another (Galatians 5:13), instruct one another (Romans 15:14), honor one another (Romans 12:10), and be kind and compassionate to one another (Ephesians 4:32).
When a person trusts Jesus Christ for salvation, he or she is made a member of the body of Christ (1 Corinthians 12:27). For a church body to function properly, all of its "body parts" need to be present and working (1 Corinthians 12:14–20). It's not enough to just attend a church; we should be involved in some type of ministry to others, using the spiritual gifts God has given us (Ephesians 4:11–13). A believer will never reach full spiritual maturity without having that outlet for his gifts, and we all need the assistance and encouragement of other believers (1 Corinthians 12:21–26).
For these reasons and more, church attendance, participation, and fellowship should be regular aspects of a believer's life. Weekly church attendance is in no sense "required" for believers, but someone who belongs to Christ should have a desire to worship God, receive His Word, and fellowship with other believers.
Jesus is the Cornerstone of the Church (1 Peter 2:6), and we are "like living stones . . . being built into a spiritual house to be a holy priesthood, offering spiritual sacrifices acceptable to God through Jesus Christ" (1 Peter 2:5). As the building materials of God's "spiritual house," we naturally have a connection with one another, and that connection is evident every time the Church "goes to church."