Good question. I would suppose at least reduced weight and reduced drag. I would suspect that pulling 6+ Gs might have something to do with it too... probably better to drop them than have them rip away from the aicraft.

"An aeronautic drop tank is an expendable and often jettisonable external fuel tank carried by aircraft for long-range flights. It is designed to be discarded when empty or in the event of combat or emergency in order to reduce drag and weight. The drop tank was first used during World War II (ca. 1942) to allow fighter aircraft to carry additional fuel for long-range escort flights without requiring a dramatically larger, heavier, less maneuverable fuselage. The German Luftwaffe began using external fuel tanks with the introduction of the ME-109 E7 and FW-190 A4 in mid-1942, after noting the disastrous range limitations of Me-109s in the Battle of Britain. The Allies commonly used them to allow fighters increased range and patrol time over continental Europe. Such tanks are now commonplace on military aircraft and occasionally for civilian use, although civilian aircraft are less likely to discard tanks except in the event of extreme emergency. The primary disadvantage with drop tanks is that they impose a drag penalty on the aircraft carrying them. A rule of thumb is that only about half the capacity of a streamlined drop tank actually goes towards increasing the aircraft's overall range, the rest going to overcome the added drag and weight of the tank itself. The use of drop tanks also reduces the number of external hardpoints available for weapons, and increase the aircraft's radar signature, both problematic for modern tactical aircraft. Some modern combat aircraft use conformal fuel tanks (CFTs) instead of or in addition to drop tanks. CFTs produce less drag and do not take up external hardpoints, however as a result some versions can only be removed on the ground." Source and further information: http://en.wikipedia.org/wiki/Drop_tank