Fillets helped with lateral stability in any situation. Helped more with the D's because of the cut down fuselage, but obviously they figured it would help with the B/C's too. The P-51's were pretty wobbly when fully loaded with fuel, at least the D's, and there was a specified order of which tanks to empty first when flying because of this. If the order wasn't followed it got worse.
And of course, as Jennings mention, better lateral stability meant better accuracy with the weapons.