Cards can be fickle at times and you never know when they are going to turn against you. Once you have a lead the best way to maintain it is to limit the opposing team's ability to name trump.
In this game we had a five-point lead, playing against a very aggressive team. The dealer, sitting in East, had a reputation for calling alone on almost anything.
I was sitting in the first seat (South). After looking at my hand, I realized I had no way to stop a lone call. My only ace was in next, the suit most likely to be discarded by the dealer. With the score at 3 to 8, it was necessary to protect our lead.
As it turned out, I was right about my assumption of there being a lone call. I had just miss-guessed whose hand it was in.
It's much better for us to have the score at 5 to 8, not 7 to 8. By naming trump here, they needed at least two more hands to complete their win. If they were at seven, one lone hand would put them out. The idea is to give your team as many chances as possible to make the points needed for a win.