Here's one paper on CNN's in APL: https://dl.acm.org/doi/10.1145/3315454.3329960

The use case is use J or K is to do advent of code or the Euler project :p

APL is a tool for thought more than a real programming language so in my opinion it's best suited to boost your puzzle solving ability.

Image processing is a fun use case. For example, here's a bit of Common Lisp using April and the Opticl library to create a mirrored meme image:

  (opticl:write-png-file 
   "~/out.png" (april-c "2 0∘{(x s)←⍺←2 0 ⋄ (⌽[x],[x]⊢)⌽[x]⍣s⊢⍵↓[x]⍨(1-2×s)×⌊2÷⍨x⊃⍴⍵}" 
                        (opticl:read-png-file "~/in.png")))

The 2 0 at the start of the APL line above controls the mirroring behavior. The second number can be set to 0 or 1 to choose which side of the image to mirror, while the 2 sets the axis along which to mirror. This will be 1 or 2 for a raster image but this function can mirror any rank of array on any axis.

April was used to teach image filtering in a programming class for middle-schoolers, you can see a summary in this video: https://vimeo.com/504928819

For more APL-driven graphics, April's repo includes an ncurses demo featuring a convolution kernel powered by ⌺, the stencil operator: https://github.com/phantomics/april/tree/master/demos/ncurse...