Guns flowing out can be used in a foreign civil war or for crime abroad, which really is the least of our concerns, particularly these days. However, an example like sending guns to the IRA when the UK is our friend creates serious political reasons to stop the flow of weapons out that will be used against allies.
Guns flowing out can also be used by an enemy against our troops, citizens, or interests abroad. Not very likely because AK's and all other weapons are readily available to most of our enemies everywhere. Still, imagine if someone knowingly sent weapons from this counrty to an active or imminent enemy. In reality, if we could swap out all our enemy combatants' full auto combat weapons with 922r compliant semi autos, that would be a good thing...
Mexico is a bit problematic for several reasons. As is often the case, these guns are involed in a triangle of money, guns, and drugs. So the flow of guns to a drug producing or conduit country/group will also invlove drug flow back in and large amounts of money going in both directions. The money the feds want to know about for tax purposes. The guns the feds want to stop because they are being used by producers and smugglers, making law enforcement in those countries less able to combat the production and flow of drugs back to this country. Add to the fact that the country is on our border, many of the guns will inevitably be used in crimes on both sides. It becomes a legit national security issue. Those same guns will be in the hands of smugglers of drugs and people who will not be happy to see border patrol or other US LE while moving back and forth. Also, when they bust the people on this side sending the weapons out, they get more information about all participants in the triangle, which leads to illegal drugs and money.