ER is an RC oscillator option, using just an external resistor - this allows you to vary the oscillator frequency over a fairly wide range - something you can't do with the internal RC oscillator. However, it does use at least one PIC pin, and could use 2 (if you use the CLKOUT setting).
You should really set all the fuse options in your source code, you don't mention what you are using to write your code, but my tutorials use assembler and set the options with a '__config' statement. It's all explained in the MPASM help file.
Just like Exo, I've never used the ER option either, I've never had the need - but it's always there if needed.