Masked Blend Mode is used for objects in which you will need to selectively control visibility in a binary (on/off) fashion. For example, consider a Material that simulates a chain link fence or grate. You will have some areas that look solid while others are invisible. Such Materials are perfect for the Masked Blend Mode.
It is important to keep in mind the difference between transparent and not rendered. A transparent surface, such as glass, still interacts with light in the form of reflections (specularity). Pixels that are culled in Masked mode simply do not draw; you will not see any reflections in those areas. If you want to retain reflections or specular aspects, you would do well to use the Translucent Blend Mode, or consider making a Layered Material.
Further, since these features don't render in the masked area, they aren't calculated at all, leading to performance savings on the GPU.
When using Masked Blend Mode, you need to pay special attention to the Opacity Mask Clip Value property. This property holds a 0-1 scalar value which controls what value of the opacity mask texture will be used as a cutoff point, beyond which all darker pixels will not render.
Opacity Mask Clip Value (Drag the slider to preview)
In the example above, the Material has its Two Sided property set to True (checked), which is why you can see the inside of the box.
Also, despite the interactive example shown here, the Opacity Mask Clip Value property is not designed to be changed at runtime.