Cause I analyzed all devices described on wiki and generally for every line of 15-pin port (except /IRQ) there is device that uses it.
Have you also checked out the other peripherals that Nocash mentions that the wiki doesn't? (edit: He doesn't think any of them use the /IRQ line either)
I don't know of any, although I have once suggested a use for it in imitating mapper 99 (I have seen the schematic for RF Famicom, and it is just wired directly to the cartridge /IRQ line), but only if the game does not use IRQ. Other uses are also possible, including actually using IRQ.