I know that God created us, but why did he put us on earth? And if he is all knowing, then why did he allow Adam and Eve to fall? What is the purpose?
Wow...I never thought that we had so much possible viewpoints in this thread. But anyway I will give my 2 cents.
As someone already stated the exact reason of why God choosed Earth instead of another planet we don't know, but we do know that we were created because we are here. Now, what is the purpose? As some people also stated the purpose for our creation is to be with God. To enjoy of all the things that God can give us. It was not because God was feeling lonely or anything like that. He just decided to create us. What was the exact feeling in the moment prior to our Creation we don't know. But maybe someday, when we are in Heaven, we may ask Him. And about why did He let Adam and Eve to sin...well the truth is that He didn't allow them to sin per se. He allow them to take their own decisions, that Free Will that we all have. And it was through that Free Will that man choosed to sin.
Hope this helps.