In the movie Body Double (1984), directed by Brian De Palma, Sam Bouchard set up the trap for broken down Jake Scully. He gave him a job of house-sitter in a grand house in Hollywood.
Jake Scully witnessed the murder of Gloria Revelle by an Indian (which was later revealed to be Sam). Police had no other option than to pressurise Jake for confession and they were doing nothing in the case. Jake tried hard and finally found that Sam set him up. In a phone call to the police officer Jake quotes
He wanted me to witness the murder, Sam Bouchard was Alexander Revelle.
(Sorry for not quoting the actaul dialogue, but he did say something like that).
Now, my question is why Sam wanted Jake to witness the murder. How would it have helped Sam in not being suspected? We may or may not know why Sam killed Gloria but I think we can know why he wanted Jake to witness his brutal acts.