Membership is important for several reasons
Membership identifies you with a local church body. As you take your place in God’s family through salvation, it is His intention that you also commit yourself to a local church and become a committed contributor in that church family. The reason for this is simple. Accepting Jesus is a personal, private decision that you make on your own. But living out your faith is a corporate, public endeavor. Our faith is not to be lived in isolation.
In addition to that, the local church is meant to be the recipient of the gifts and resources that God has entrusted to your stewardship.
And finally, membership provides voting rights and decision-making influence as this local church seeks to fulfill the mission that God has given it.