It encourages people to actually join the fellowship in their minds and in their hearts.
It avoids the old “join us for 6months before you serve”… as if going to 15+ sunday meetings is going to really help them understand what your church is on about, our help you know enough about them?
It allows people to opt-out early, rather than wasting 6-18months in your church, before they realise you believe in predestination, and they leave; that’s just wastes their time and your church’s time too.
It allows people to opt-in early. If people join you well; understanding all the things your church thinks is important, agreeing with you, and keen to share the vision you’re praying for… then, let them serve, get them involved.
It gives them an opportunity to meet the pastor/staff that just doesn’t happen easily on a Sunday; there really is so much happening on a Sunday, and if your pastoral staff are excellent, then the’ve probably at least said hi once… but a membership course allows them to have a conversation – maybe the only one they’ll get in years (depending on your church size).
It gives them enough information to start asking good questions. Most people don’t have questions about your church, because they don’t know your church, they don’t have any buckets of thinking to investigate. A course helps them see that they have questions/issues.
There’s probably many more…