In Post-processing, it is nearly impossible for a designer to avoid using the image masking features and methods. Image masking opens up a new window of endless editing effects and a dedicated designer is bent on taking every single opportunity.
Sound knowledge about these options and functions will ensure a satisfying end result. Now to address the question at hand:
Non-Destructive: As opposed to erasing a background using the Eraser Tool, masking technique does not obliterate the image details. They are cleverly hidden below various layers so that they can help us out in case we need to make changes. On the contrary, the Eraser tool permanently deletes these pixels and it is close to impossible to bring those back in case a tweaking is required.
Transitions: The basic or simplest function of image masking technique is to have a "hide and seek" effect in some areas of the photo. This transitioning effect can be created using brushes and gradients for soft masking. This requires delicate strokes and soft brushes. This transparency of pictures can be controlled. The opacity level can be adjusted to suit the photo and its background. This is not the only technique for achieving this effect, but it is the simplest.
Editing Specific Areas: Many times we are faced with projects where we need to edit a small portion of the photo; such as, changing the color of someone's clothes in a photo and fixing shadow/light issues. You can use masking techniques to highlight the portion and edit it as you wish e.g. color correction, brightness, contrast, exposure, shadows etc.
Removing / Replacing Background of Translucent Objects: Masking is an easy option when it comes to removing backgrounds of translucent objects. Any object with any level of transparency can be isolated from its background by careful masking. Even in cases of semi-transparent clothes' photos, this technique can be applied.
Single Advantage of Clipping Mask: Clipping mask, when compared to Layer Mask, has the advantage of making different areas visible by simply moving the clipped image. It can be determined by the user which part of the background they want to be visible and which part they don't by using clipping mask. Other than this one advantage, regular layer masking is more than good enough for most masking work.
Creating Collage Photos: Collage images are fun and it is even more interesting when you play with the masking tools while making a collage. Interesting and cool effects can be made by using a number of pictures and masking them. Soft brushes in varying gradients and hues of gray will definitely make these blending smooth.
Photoshop techniques have no ending possibilities and features to manifest your wildest imaginations. The same result can be achieved by using many different tools and features in Photoshop. Each user is skilled in each technique which is what determines the ultimate output. Which technique you want to use is entirely up to you!