Why would someone use "IsLocalFlag" instead of "IsFlag"?

I was trying to create an assassination quest in my game, which involved NPC A giving the PC a contract to kill NPC B. Once the PC receives the quest I wanted a new dialogue option to appear when speaking to NPC B. However, the localflags I put on NPC A's dialogue didn't work on other NPCs dialogues so I switched the IsLocalFlag conditions to Isflag. Now it works.

But what's the benefit to using a localflag then?