What is purpose of exporting system using tcl scripts generated by Qsys?
The TCL file is an alternative to the Qsys file. It allows you to use the system as a subsystem in Qsys.

So why would you want it? Because you can edit the file as a general TCL script, allowing you to add more parameter options that can't be done in Qsys graphically.

For example say you want a system with N submodules - that can't be done in Qsys, but it can be done in the TCL file with a for loop.

As another example, say you have a system that can select between different subsystems depending on a parameter. Again you can only do that in the TCL version.

You only need the TCL version if you are going to use it. Otherwise there is no point.