Why Do You Need Another File Manager Instead of Finder?
Finder is a graphical file management application. Outside of the main folder viewing section, it also has a sidebar (shortcuts can be added to it) and a toolbar. If you want to browse several directories at once, you need to launch a separate instance of Finder. A different file manager for Mac will often include a better alternative, such as tabs or multiple viewing panels.

It’s common for people to store large, currently unneeded files on a local server. It’s like a disposal bin - a decent way to free some hard drive space without having to use external drives or just straight up losing your data. Well, Finder can’t access these. FTP and SFTP support are a step up, a much-needed one.
Archive handling is lacking on Mac, unless you install third-party software. All you get is ZIP and a proprietary Apple format, with no way to change the compression level. RAR files are common, so a file manager on Mac that can unrar is a good replacement for Finder.
There are countless other reasons. Customizability, the lack of file access history or an operation queue, no cloud service support, no sudo, and other failings - that’s enough to make anyone consider getting a new app.