I've heard this before but not certain I recall the answer. But I'll take my best guess anyway, did it have to with facilitating carrier landings or working with them on the deck? IE, allowing men to easily walk under the wings? Seem to recall something about that. There's an embedded youtube video of an Air show from Dover AFB on the first link tanknetter posted. A good one.
“Do you know why the wings were shaped that way?”
The F4U incorporated the largest engine available at the time, the 2,000 hp (1,490 kW) 18-cylinder Pratt & Whitney R-2800 Double Wasp radial. To extract as much power as possible, a relatively large, 13 ft, 4 inch (4.06 m) Hamilton Standard Hydromatic three-blade propeller was used. To accommodate a folding wing, the designers considered retracting the main landing gear rearward, but for the chord of wing selected, it was difficult to fit undercarriage struts long enough to provide sufficient clearance for the large propeller. Their solution was an inverted gull wing, a similar layout to the one used by Germany’s Stuka dive bomber, considerably shortening the length of the main gear legs. The anhedral of the wing’s center-section also permitted the wing and fuselage to meet at the optimum angle for minimizing drag, without the need for wing root fairings.