So you can learn about God, Jesus, and Christianity.
The Bible commands it.
(Hebrews 10:25 NIV) Let us not give up meeting together, as some are in the habit of doing, but let us encourage one another--and all the more as you see the Day approaching.
(Matthew 18:20 NIV) For where two or three come together in my name, there am I with them."
If you're already saved you won't grow as a Christian unless you go to church. There are blessings that God gives to those who are obedient.