Easter egg: To access the chronological version of the film on the 3 DISC SPECIAL EDITION (with the negatively inverted photos on the cover), insert DISC 2 and highlight BIOGRAPHIES, the press right and the cursor will hang over a black space, press enter and the feature will start. On this version, chapter skips and scans are ENABLED.
Easter egg: On Disc 2 of the limited edition DVD, select the Clock icon on the main menu. When you get a series of questions, pick C for all of them, until you get 4 pics of a woman changing her tire, then put the pics in reverse order. If done right, the credits will start to play backwards, then the movie plays in the correct order with the phone call scenes being first.
Easter egg: To get to the chronological play of the film on the original (single-disc edition), go to the special features (with the polaroids spinning towards the screen), and press the select button when the blank polaroid is filling the screen.
Answer: It is never explicitly given. The most Leonard says on the subject is: "I'd rather be mistaken for a dead guy than a killer." Speculations include (you can make up your own motives as well) : (1) The clothes and car are so much nicer than his. If you are willing to kill someone: stealing is not really a "crime." Why not take the nicer objects? (2) It could be part of his "routine": Kill a man, take his clothes and car. The clothes he had on and the truck may be from the man he killed a year ago. (3) It could be that he wants to make the killer of his wife suffer even more, and takes his clothes as a way of humiliating him. Leonard takes the man's life-his clothes and car, which are wrapped up in his identity-just as the man took his. This idea seems to work with a theme in Memento about "Identity" (especially mistaken identity). Natalie thinks Leonard is Jimmy, then thinks he is Teddy, then learns he is Leonard. Teddy is "mistaken" for the second killer, Jimmy is "mistaken" for the 2nd killer. Sammy's story as a part of Leonard's story, etc. (4) It could "simply" be explained as a "plot device": Leonard has to do it, otherwise he won't find the note in "his pocket" and meet Natalie. (5) Leonard doesn't want to admit he's a murderer. He's lying to himself. If he's the victim, then he cannot be the murderer. (6) Leonard takes Jimmy's clothing as part of his routine of killing J.G.'s he becomes another person, he's the victim not the killer, thus "I'd rather be mistaken for a dead guy than a killer." and that's why he also takes his car, so he has to, once again, find his wife's killer and kill him.