I like to occasionally appear, though usually it's when friends remind me Allegro is still a thing.
So a decade ago, I had a need Allegro could have filled: General purpose raster graphics library. No, not cross platform, but general purpose. Give it a chunk of memory, and here are a library of function for doing anything you could ever want (sprite wise) in that RAM. Hey maybe even some simple 3D too. I would have loved to have it as a fallback renderer, when a GPU wasn't available. I still think there's a distinct lack of a good (or any) general purpose fast raster graphics library. And heck, if there was an API variation you could feed a single triangle/quad render function too, it could be forward compatible as well. Would have been great for PC/Console/Obscure porting. Alas.
When I was a wee-lad using Allegro for my gamedev on DOS and Windows 95 machines, it was the raster graphics performance and features that made me use it. Before Allegro I was a big user of a Turbo Pascal library I think was called SPX. It was capable of rendering full-screen 2D graphics at 60 FPS on my 486 DX computer. I didn't understand what or how it was so capable (I didn't understand Assembly yet), but it was easy to make, load, and display sprite graphics. It came bundled with a tool SPXMaker, and that was my workflow (edit sprites, export, load, draw in game). It felt good.
When I eventually moved to C, Allegro filled the same niche. I paired it with a similarly useful sprite and package editor, the name of which I can't remember (I never found anyone else that heard of it or used it). I believe I had a Pentium 90, which was like infinite CPU power back then. I would dabble with other libraries and tools, but Allegro is the one that performed best (at least on my teenage budget of $0). Screw Direct X, I had DJGPP and this monster.
To me, Allegro was a complete C workflow for making, packaging, loading and displaying sprite graphics on screen fast.
Eventually I start doing Nintendo GameBoy homebrew. Lo and behold, the community has an amazing sprite and map editor freely available. My workflow stays intact (make assets, load assets, display assets). Before I know it, I'm making commercial GameBoy games.
A friend starts Ludum Dare. I champion Allegro as a viable tool for rapid development, because my workflow. Easy to make, load, and display assets. I use it for years, most of the early years of LD. It's me and my workflow.
At some point I stop using Allegro. I had worked on nearly a dozen console games, 4 of those in assembly, and I was always the tech lead (doing the low level shite nobody else wanted to). It just doesn't feel right anymore. I'm thinking of high resolutions and bit-depths, file format conversions, PNG files, and whatnot. The Paletted BMP and PCX formats just don't cut it anymore. SDL becomes my new home.
The thing I lost moving to SDL: Fast Raster Performance, and the workflow. The workflow was already gone. I think my mystery Sprite Editor was a DOS DJGPP program, and I was a "Paint Shop Pro" user now. Switching to a fullscreen DOS app really felt clumsy (though it had basic animation support, or rather, a button I could push to toggle frames). It's the mid 2000's, and nobody runs DOS apps anymore.
SDL quickly solved its inadequacies by having native OpenGL support. Suddenly I could do stuff again, faster than ever, with free scaling and rotating support. I still lacked a great workflow for making assets. I had an okay one, painting large images and inputting numbers (regions), but nothing so clean and optimal ever again.
Basically: If Allegro did something with that heritage of being a great sprite library (or even a full workflow), it would still be useful. I don't know what A5 is trying to be. Nostalgic? Newstalgic?