Originally it was thought that the Falcon Heavy landing would use some mechanism to separate in time the landing of the two core boosters. Perhaps, slow one down, allow one to reenter a bit faster to buy enough time for one to land, perhaps let the dust settle down, before the second one landed.
Elon tweeted that he expects them to land simultaneously.
The contingency pads in the EIS apparently have yet to be built, as late as the CRS-9 landing at LZ-1. So it is unclear yet what the purpose would be, although as a divert location it does make sense, as is stated in Pearson's answer.
An article in the Orlando Sentinel indicates they are looking for permission to build two more pads. It is unclear from the article if they mean the contingency pads, or more pads. The article references within the LZ-1 complex, but never calls them by the contingency pad names.