Church leaders ask this question to measure their results. It allows them to calculate whether it's worth the time & money to continue the effort.
Consider this scenario:
2,000 gospel tracts are distributed with a clear message of God's redemptive plan of salvation. The result is only 1 person visits the church-house on Sunday morning.
Now contrast it with the follow scenario:
2,000 flyers are distributed with an invitation to a pizza party that includes a bounce-house for the kids and a free gas card for mom and dad. The result is 1,500 people visit the church-house that Sunday.
Comparing these 2 scenarios, you can see why asking, "Did the outreach work?" is the wrong question. It's the wrong question because BOTH worked. One attracted people with a carnal method. The other attracted people through a spiritual message. I'm not looking for a better method, I'm looking to reach people with the right message.
The Bible tells us we are to do all to the glory of God. Instead of asking, did the outreach work, maybe a better question would be: was God glorified during our outreach efforts?
You can reach people with all types of alluring gimmicks. You can use carnal means to draw people in, and it will work. But it will give you carnal results.
I'd rather preach the unsearchable riches of Christ, allow God to provide the increase, and give Him the glory for the spiritual result that He provided.