"I find it hard to imagine why one would drift so far of the localiser when using a flight director coupled to it."
If it happens just one time, and being in raw data helps you catch it......
It would be unlikely to see such an event in the typical intercept scenario. But what about a busy approach controller trying to squeeze you in (and you accept it), a busy cockpit (late accomplishing checks), mountain environment, autopilot on, approach gives you an intercept heading but the bank selector is in 10 degrees instead of auto (you shoot through the LOC), it's 0800 but your body thinks it's 0200 (flew through the night), ....