The author talks about windshields in the last paragraph to:
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to offer evidence that insect populations have declined - The mention of windshields serves as a metaphorical representation of the decline in insect numbers, as fewer insects hitting windshields indicates a decrease in their population.
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to dispute a claim from paragraph 2 - The reference to windshields can also be used to challenge or provide evidence against claims made earlier regarding the abundance or health of insect populations, reinforcing the argument of decline.
In summary, the focus on windshields helps to back up the assertion of declining insect populations and counter earlier statements about their status.