Well, a claim refers to the extent of the protection conferred by a patent. If claims are so important so why they are not mandatory for the provisional application?
There has to be a reason for such a lenient attitude towards mentioning a claim.
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I am not a lawyer, but I think it is because the provisional application is not the application that gets examined. It needs only to sufficiently disclose the invention so that the non-provisional application can then be examined and claim the earlier priority date. Remember, claims get edited all the time as the patent examination process progresses. This doesn't mean it isn't a good idea to consider what your claims would be or even to include them in a provisional application.