It’s not an oddity. There is purpose in it. How many states other than Texas do you know of that could be virtually self-sufficient?
As far as electricity goes, Hawaii and Alaska have their own electrical grids. Something tells me that Texas is not self-sufficient in all respects. For instance, even in electricity generation Texas uses more coal than it produces. No doubt most pieces of land of that size could be run in a fairly self-sufficient manner, especially if blessed with energy resources as much as Texas.
Interestingly, another area with frequent mumbling about secession also has it's own grid: Quebec.