A viewfinder on a mirrorless camera is useful for many of the same reasons photographers prefer a DSLR viewfinder over the rear screen.

Main advantages of an EVF over the LCD:

• Easier to see in bright sunlight, where the rear screen can be washed out.
• More stable shooting: pressing the camera to your face adds a third point of contact, which helps steady the camera, especially with longer or heavier lenses.
• More comfortable and immersive: the viewfinder fills your vision and blocks distractions.
• Lower power use than keeping the main rear display on continuously, so battery life can improve.
• Diopter adjustment can help users who don’t want to wear glasses while shooting.

An EVF is especially helpful for action or distant subjects, such as birds in flight, where glare on the rear LCD can make framing difficult.

That said, the rear LCD can still be better in some situations, but if you often shoot outdoors, use longer lenses, or want better stability and visibility, a mirrorless camera with a viewfinder is usually the better choice.