openjdk (java) is needed for the Eclipse Nsight Edition.
If you’re asking for a detailed description of what each and every dependency is about, you probably won’t find that.
The package manager method installs using dependency chains, so that if you already have a particular dependency, it does not need to be reinstalled. For a machine which is largely up-to-date, this could save a lot of time.
The package manager method offers two different packaging options, one which is a “local” and one which is a “network” install. The network install uses a set of network repositories. The local method is effectively “what the runfile installer does”, in that it contains all of the necessary packages in a local repo/archive. As a result you get to download a large ~1.5GB blob, just like you do when you download the runfile installer.