Why is the airport beacon lit?

A beacon in daylight

Airport beacons normally run from dusk to dawn. The AIM (2-1-9) says that in Class B, C, D and E surface areas, a beacon operating during daylight often means the ground visibility is less than 3 miles and/or the ceiling is less than 1,000 feet. In other words, the weather may be below VFR minimums.

Don't rely on it alone

The AIM also says pilots shouldn't rely solely on the beacon to decide whether the weather is IFR or VFR. Daytime operation isn't required, so check the weather report.

The flash pattern doesn't change

A lit beacon keeps its usual flash pattern. For a civil land airport, that's alternating white and green. A military airport beacon is told apart by two quick white flashes between the greens, not by being on in daylight.