Paul left behind an open door in Troas. If you hold certain views about guidance you might think "that was it - he blew it. God opened the door and he missed it." I know people who think about their lives like that, always looking back to what they consider to be the opportunity for serving God with their lives that they turned down at the time and have regretted forever after. And then spend the rest of life assuming they can't have another go.
It really isn't like that with God, and here Paul explains why. He uses a picture of God being the King who has conquered him, and us. He is the victor, we are his captives. Christ has beaten death, overcome the resistence of rebels and captured us to his cause. We are on display as captives in his triumphal procession, his trophies, his possessions, his spoils.
Like any conquering general he now has the right to disperse and use his trophies however he likes to accomplish his goal. That goal is to spread the Good News, not just in Troas, but everywhere. Paul is committed to doing what his chief requires and so discovers that leaving the open door in Troas wasn't the end of fruitful service, it was just an opportunity to do it elsewhere.
This is the vital point to grasp: what we do comes from who we are. v15 says that our lives are a fragrant offering present by Christ to God. The best verse for helping understand this is Eph. 5:2 which says that Christ was a sacrificial offering, taking away sins, and was fragrantly pleasing to God.
And we are caught up with Christ. We are the goal of his fragrant sacrifice. We spread the good news because our fundamental identity is "Captive. Fragrant-Aroma-to-God". What do captive-fragrant-aroma-to-God people do with their lives? They spread the aroma, obviously. They can't do anything else. Aromas spread and pervade, that's what they do. If a perfume doesn't spread so that people can't smell it and be attracted to it, it isn't a perfume.
When we live in the service of Christ, sacrificing ourselves in order to extend his love, grace and forgiveness this in turn delights God like a beautiful fragrance. We have become the aroma of Christ's own sacrifice. This is the answer to why Paul didn't blow it by leaving Troas. The perfume wasn't just good for Troas, it worked everywhere.
Of course there is a way to blow it. Which is to say "I missed it back then, and I'm not really interested anymore." That way we do step out of the specific guidance God has for our lives by denying our staus as captive-aroma-of-Christ people. We cease to see ourselves - and display ourselves - as trophies of his grace. We stop looking for ways to ways to be like perfume. We no longer consider ourselves his captives and him our conqueror. When this happens we should no longer expect to be used by God wherever we go.