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In QGIS I just used the tool "cliprasterbymasklayer", which produces a new raster layer as output and gives me the option to "create an alpha band". I read that the alpha band represents the transparency but I don't see any use case. Even if I don't choose this option I can make the new raster more transparent. If I check the option the new raster is lighter and has 0 as lowest value.

I was just wondering about a situation in which that could be practical. Any ideas or examples?

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    While list compilation questions exist in the GIS SE archive, most of those questions are locked, since they're now too discussion-oriented for the matured Q&A database. This sort of question would be very appropriate for Geographic Information Systems Chat, if it were more heavily utilized (less neglected). – Vince Dec 28 '19 at 12:26
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That option would be useful when you want to clip the raster by a non-rectangle polygon.

Let's say you have an oval polygon and clip your raster with it.

enter image description here

The raster will be clipped to the extent (above: black rectangle), but you need to mask the area outside of the polygon (above: blue ellipsoid) for which the "create an alpha band" option will work.

enter image description here

For instance, it will give 255 for the inside cells (blue) while 0 for the outside ones (yellow). (Values depend on the raster data type). If 255 is set as NoData, cells inside of the oval will become invisible.

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