Jesus said that He was saying these things so His disciples would not be offended. Jesus was preparing them for what was to come.
Why speak of preparing them a mansion in heaven? The reason for this was to comfort the disciples and help them put things in perspective. In 1 Thessalonians 4:18, Paul tells us to comfort one another with words about being gathered unto the Lord in the air.
Paul said again, "I reckon that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us" (Romans 8:18). Someday, all of our trials will seem like nothing and this can be a great comfort to us now.
When we think about being with the Lord through all eternity, it helps us to put things in proper perspective. It is easy to get fearful about our problems and think all is lost. However, for those of us who are born again, if worse comes to worse, we still have the promise of Jesus wiping all the tears from our eyes and preparing a habitation for us where all our former sorrows will have passed away. This keeps us from despairing and makes us much stronger in our faith.
In heaven, there are many dwelling places and Jesus is preparing one for us.
The thing that is going to make heaven "heaven" is the fact that we will be with Jesus. No doubt, there will be things to see and do that will be wonderful, but nothing will compare to being with the one who loved us and died for us.
