For one, you are now not limited to 'old' technologies like COBOL and PL/1. You can actually write Perl and PHP programs using USS. You also have available all the utilities available on Unix like SFTP and Apache.
The only caveat to this is that you will need to pre-define a file to be used by your ID in USS. Your Unix programs can only use the files within the USS file system. If you need to use any z/OS files, you will need to 'transfer' it to USS before it can be used.
Another thing to consider is when you are using USS to do your SFTP file transfers. You will need to pre-define your space to the largest file you will need to transfer. If you are transferring a file that is 2GB in size, you will need to preallocate at least 2GB of space to be able to copy the file from z/OS to USS. The good thing is the space can be archived when not in use to free up space when it is not needed.
So What Can You Do With USS?
You can access your mainframe USS files like you would on a Windows network drive. SMB has been around for a while in the Linux world. Windows desktops can access Linux files using SMB. When installed in USS, Windows desktops can also access USS files using SMB.
Another thing available on USS is its HTTP server. It is an Apache server so it can serve web pages on the mainframe. You can use this as your web server. Imagine hosting a web server on the mainframe with all its processing capabilities.
One of the strengths of the mainframe is its security. Mainframe security is probably the oldest and most mature security software around. Being part of z/OS, USS also taps into the mainframe security system.
You can also leverage tools available on Unix for your mainframe processes. Unix tools were written to do what they were designed to do extremely well. Tools like awk and Perl allow developers to develop quick and efficient code for parsing and reporting. You can send your z/OS files to USS for processing and then get them back for further processing.
This is the best of both worlds. The processing power of the mainframe, the stability of the operating system, the security and maturity of the technology, plus the robustness of Unix all in one box. It is worth a look.