not forsaking the assembling of ourselves together, as is the manner of some, but exhorting one another, and so much the more as you see the Day approaching."
Hebrews 10:24-25 (NKJV)
Growing up in Wisconsin, every year in the Fall different birds could be seen overhead migrating to some destination in the southern part of the USA. There they would spend the cold Winter months before returning to Wisconsin or some other part of the northern USA. One thing you can see with migration is that birds migrate in flocks and you usually don't see birds migrating on their own, they normally travel together. There are several reasons for this, but it basically comes down to the fact that there is a value in being together that is not enjoyed being alone.
People, even Christians, sometimes ask, "What is the point of going to church? I can be a good Christian by myself at home". Unfortunately, while this question deserves an answer, when someone asks it, they show that they don't understand the value of the local church. There is a value in being together that we cannot enjoy if we are alone. Our relationship with God is a personal thing, but we were not meant to live our life alone. Believers make us the Body of Christ and every part of the Body of Christ is important and needs the rest of the Body to function properly.
Hebrews 10 talks about stirring up and encouraging one another, this is one reason it is important to meet together with other believers. You can't encourage someone you never met and don't know. You can't give something to someone you never meet or tell them something if you never see them. Through building relationships with other believers we put ourselves in position to be a help to them and them to be a help to us. Going into a church building on a certain day of the week is not what makes you a Christian, but going to a church on a regular basis can greatly help you in your life as a Christian.