There's no difference, but it does avoid a grammatical pitfall. Consider:
A large number of people is/are standing in front of the cinema.
"Are" here is, according to formal grammar rules, wrong, because the subject of the sentence is "a large number", which is singular. However, "is" simply sounds wrong because it comes directly after a plural noun. If you start the sentence with "large numbers", it avoids this problem.
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