Why attribute alt is important?

alt attribute is still mostly missing, misused or misunderstood what is for. Let me summarise it why alt attribute is important.

1. Adding alternative description of the image is one of the most important principle of web accessibility. This allow users to understand the image context when using assistive technologies, e.g. screen reader.
2. alt content is visible while image is loading.
3. alt content is visible when image is not available.
4. alt content is visible for text-only browsers.
5. alt content is important for search engines (e.g. Google, Yandex, Baidu) as it helps to understand the image context. This is related to SEO.

Note: an empty alt="" attribute tell the assistive technologies to ignore it. It means that the purpose of the image is decoration.