The mirror is for the small optical viewfinder window at the top. When you hit the button for live preview, the mirror is “locked up” and the main sensor powers

The mirror is for the small optical viewfinder window at the top. When you hit the button for live preview, the mirror is “locked up” and the main sensor powers a “live view” on the LCD screen which gives you a preview including aperture/iso/exposure/etc settings all applied just like on many mirrorless cameras. The reason the small viewfinder exists is because it draws power to keep the sensor on and the mirror locked up. Keeping the sensor on for long periods of time can cause the sensor to heat up which will degrade picture quality (albeit not by a lot with current high end sensors, so the main reason is power draw). Historically the focusing and E-TTL sensors were also only on the same side of the mirror as the optical viewfinder, so you’d lose auto-focus and flash metering when the mirror was locked up, but that is generally not the case anymore.