Because you have an option to use less than all if so desired. Otherwise just Import from Catalog and get all images imported.
Don't do this, all your presets are not there on other machines or newer presets don't travel back to the original. They are on the other machine deep in a system folder.
Yes, exactly, for folks combining catalogs, just simply import the catalog. This is the OP intent I think. For subsets of a catalog, then export the selected as a catalog and import the exported catalog and images.
And for the OP's task at hand, combining whole catalogs, I think they would need to move all of their current catalogs and images to the new computer and make sure the images are linked up. If there are drive letter changes the images will not link up auto-magically. Once everything is moved, then create the new master catalog and import the old catalogs into it. If the images are in separate locations per old catalog, they can be moved to a common new location after they are imported into the new catalog.
For presets with the catalog, backing up and portability (I'm guessing the catalog, presets, and images are on a external drive) seem to be the two benefits.
The OP doesn't seem to be in a portability situation, seems to be loading them on the computer. Cloning presets to multiple drives/catalogs will inevitably result in a situation where each will have changes and then it becomes a mire of how to sync them up. The default location may be a better choice.
For Back up purposes, hopefully folks are instituting a good back up regimen. If they are only focused on the LR assets, they probably aren't doing themselves any favors.