what's the purpose of finder

Finder shows files and folders and allows you to move, copy or otherwise manipulate them, as well as install and launch software.

If you look under Help on your menu bar, you can open the Mac User Guide which explains completely how to use Finder.

You can also attend a free class at an Apple Store on using Finder in the MacOS.

If you are familiar with Windows, Mac's Finder is analogous to Windows File Explorer.

The Finder is the visual abstraction interface between you and the UNIX operating system found in the Terminal.

Use the Finder on Mac - Apple Support