Bradbury uses the Hadley family in order to illustrate what in "The Veldt"?

The family is used to demonstrate how people have become too dependent on technology.

Technology can make our lives easier.  However, there comes a point when it runs our lives instead of us using it.  Bradbury is using the clueless Hadley family to demonstrate that.  They have outsourced everything in their lives to technology, including the care of their children.

The Hadleys are clearly disturbed by their nursery.  They spent a fortune on it, and it is state of the art.  It seems to telepathically communicate with their children.  On its walls their innermost psyche is revealed.  It is a frightening revelation.  The children have very dark minds.

Mrs. Hadley is frustrated because the house has taken over her role.  She no longer is mother to the children.

"I don't know - I don't know," she said, blowing her nose, sitting down in a chair that immediately began to rock and comfort her. "Maybe I don't have enough to do. Maybe I have time to think too much. Why don't we shut the whole house off for a few days and take a vacation?"

Mr. Hadley does not seem to understand his own children.  He brings in a psychologist to find out what is wrong with them.  Although the psychologist is a person, the bringing in of this man demonstrates another attempt by the Hadleys to distance themselves from their children.  They do not try to solve the problem themselves.

In the end, the Hadleys are destroyed by the technology they have let into their homes.  Their children have become monsters, and use their technological marvel to turn on their parents.  The parents never really saw it coming.

"Don't let them switch off the nursery and the house," he was saying.

Mr. and Mrs. George Hadley beat at the door. "Now, don't be ridiculous, children. It's time to go. Mr. McClean'll be here in a minute and..."

And then they heard the sounds.

The family demonstrates to the reader why it is important to make human connections.  The more disconnected we get from each other as we rely on technology, the harder it is to reconnect.  Eventually we do not realize what we have lost until it is too late.