Weirdly I know about this category of software from a prior life working at a major CMS company.
So, "digital asset managers" (DAMs) are basically fancy versions of the WordPress media library:
https://wordpress.org/documentation/article/media-library-sc...
Why might a fancy version be needed? Well, the basic media library bundled with WordPress just uploads files directly into a web server directory, and then serves them from your web server using its static HTTP serving configuration (e.g. typically via Apache HTTP Server). The uploads are simple and 1:1, whatever you upload through the browser ends up on the web server as a file.
But usually, a media library could "do more" with those assets -- it could resize them, convert them to web-optimized assets (e.g. WebP), could create thumbnails and crops for different page layouts, could serve the content from a CDN. And that's just images, but there are other things you could do for PDFs (create a searchable index), MSOffice files (auto-convert to PDFs), or videos (extract thumbnails, re-encode in other resolutions, serve from video CDN or hosted video player service, like Wistia or Vimeo).
Of course, if you don't use WordPress, perhaps your CMS doesn't have a media library function at all, in which case a DAM might be even more helpful. This seems to be the case for a number of static site generators (SSGs), for example.
On a quick glance, it looks like Daminik is going after this with a separate open source project (written in PHP), and for its alpha is focusing on image management (using ImageMagick under the hood for image conversions). It also seems like it has built in support for uploading assets to Amazon S3, which would mean presumably you could very easily put the AWS CloudFront CDN on top.
(I run a personal WordPress site, and the way I personally handle this is to live with the built-in WP media library, but then put Cloudflare caching and CDN atop at the edge.)