Answer
The silvered surface reflects the light of trailing cars to the driver in the "daytime" position. The front surface of the glass does the job in the mirror's "nighttime" position.
Work Step by Step
When the mirror is tilted upward in the "nighttime" position, the light from trailing cars reflects from the silvered surface and bounces to the cabin ceiling. Meanwhile, the driver sees the light reflected from the front glass surface of the mirror.
The front surface reflects about $4\%$ of incident light (page 523), which is enough to see cars and their headlights in the rearview mirror during nighttime driving.
