To put numbers to it, according to google there's a gravity strength max discrepancy of 0.7% between the places with the lowest and highest gravity on the planet. Assuming using ~17g of beans for a coffee, that comes out to an error of up to 0.12g. Seems kind of dumb to correct for 0.12 grams, in the absolute most extreme case. If they just manufacturer calibrated the machine for "average" gravity then there'd be a max of +-0.35% difference in gravity, for a maximum error of +-0.06g for a 17g brew.
Quite a lot, at least at the weight and speed scale a missile operates on. The biggest source of gravity force difference is the distance between the equator / pole [1], and altitude (both in terms of altitude in reference to the planet core as well as altitude in reference to ground, e.g. due to the mass of a mountain range) also adds jitter [2].
On top of that, dead-reckoning is disturbed by aerodynamic effects such as wind, as well as drift from the gyro compass itself... a fine-detailed gravity effect map can be used to compensate for both of that, and adding imagery maps and radar/laser distance measuring is even better.
To add even more context... that is the reason why Germany's Chancellor Scholz is refusing to deliver TAURUS long range missiles to Ukraine. TAURUS owes its significant increase in precision over Storm Shadow / SCALP (UK/FR equivalent) to that kind of sensor aids, but the assistance dataset aka the photogrammetry, gravity and height maps of the flight route must be individually created and loaded into the missile as part of the mission preparation. As you can imagine this kind of precision information is stuff that we really don't want to get leaked to Russia so we can't (or don't want to) deliver the systems for programming to Ukraine where it might get stolen/diverted to Russia, so Scholz is afraid that the necessary direct cooperation of German Bundeswehr soldiers with Ukrainian soldiers might be seen as an act of war of Germany by Putin. (Complete horse dung if you ask me, but I can understand where he's coming from)