I don't use bat as a replacement for cat, but as a replacement for less. Sure, bat can concatenate a file, but I'd say that's not the main use. The syntax coloring of file contents is my favorite use case. Line numbers and some of the other options are useful at times too. Also, I don't think I've ever needed to concatenate multiple gigabyte sized files (yes, I am sure it exists for a lot of people, but would say it's not as common as looking at a file).