Alt tags are a text alternative for images when a browser cannot display them correctly. Like the title, the alt attribute describes the content of an image file.
Today, these alt tags are not the deciding factor for your SEO value. Still, they are beneficial from a usability standpoint. An important factor that Google also takes into account. If your image does not load for any reason, users will still get an idea of what it should contain, thanks to your alt tag.
It's handy for visitors who use speech software - due to higher age or a visual impairment, for example. The speech software bases itself primarily on your alt description and thus tells the visitor what's on the website. Especially for governments and membership organizations, this is an important aspect to consider in the often mandatory inclusive website design.
So, first of all, write relevant alt descriptions that effectively describe what's in the image. Be careful! Also, don't stuff them with keywords in the hope of ranking better. That's "keyword stuffing" and against Google's rules.