Things I find myself doing via tcl are:

• Analyzing a script. I.e. printing info about selected nodes, such as the number of Write nodes along with their current state (enabled/disables) and their render order.
• Looping through nodes to change multiple knob values at once or simply display their available knobs
• Extending the functionality of nodes (i.e. make a crop node conform to the current viewer's roi)
• Creating import/export functions to communicate with other programs or use external files to create nodes or animation curves
• Reading a database to make specific info available in Nuke
• Set script defaults based on environment variables

etc...

Some standard features and tools in Nuke are in fact nothing more than tcl scripts so it's a good idea to look at the default tcl files that ship with Nuke to get an idea how they work.

Some of these tools are:

• "File/Goto Frame..." calls goto_frame.tcl
• "File/Buffer Report" calls cache_report.tcl
• "Edit/Node/Info viewer" calls infoviewer.tcl

Lastly, the menu.tcl which can be used to customize Nuke, is another good example for how tcl can be used in combination with Nuke.  I highly recommend getting familiar with how to set it up, it'll be your best friend when you want to add and change menus, hotkeys or default values for nodes (check out pages 137-139 in the Nuke_User_Guide.pdf for that)