You wouldn't, but if you have the "white" fog lights like I do, having them on with the high beams brightens the area right in front of the car, which can help out with visibility on clear nights.

I re-wired my foglights so they can come on with the parking lights or the high beams; was very worthwhile IMHO. Also, at least on my '85, I'm 99% sure that the socket doesn't even exist for the "rear" foglights...I wonder if the socket can be retrofitted in? Or are they illegal to use in the US?

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2001 VW Jetta TDI, 5 speed, daily driver
1991 Ford F-350, work in progress
1984 Ford F-250 4x4, 6.9l turbo diesel, 5 speed manual
Previous oilburners: 1980 IH Scout, 1984 E-350, 1985 M-B 300D, 1979 M-B 300SD, 1983 M-B 300D
Spark-free since 1999