It's first time I heard of buildpacks, can I just throw in other than nodejs, ie. Laravel source code and build an image?
What you're looking at here is informally the "v3" effort, which extends and consolidates the "v2a" and "v2b" designs evolved by Heroku and Cloud Foundry respectively from the original Heroku design.
In v2 both Heroku and Cloud Foundry provide supported PHP buildpacks, as well as Java, Ruby, Python, .NET Core and I forget the rest right now. There are hundreds of community buildpacks.
What is the rationale behind having the build step tied to the packaging format?
Typically this means that buildpacks look for files that correspond to the relevant ecosystem. Maven buildpacks look for pom.xml. PHP buildpacks look for composer.json. Etc.
Nothing in this creates a hard binding. Detect steps may use whatever logic they need to decide on whether to signal they can work on a codebase.
Edit: in the v3 design the detect script can also provide dependency information that later steps can pick up. So for example, a JDK buildpack can say "yes, I can interpret this codebase, and I can contribute a JDK". A later buildpack can then look for this contribution as a condition, eg. the Maven buildpack can say "I will proceed if I see a pom.xml and if there is a JDK available".