It sets the bait for hooking scanners who become readers who then turn into responders. The opening sentence can be both a hot spot and a rough spot. It's a hot spot for the reader because it's one of the first places the eye looks for the answer to the question, "What's in it for me?"
It also can be a rough spot for the writer for all the obvious reasons. There's a lot of pressure to get it right, and we want to get it right the first time, which can be challenging and stressful. So, for writers, the opening sentence can be a very rough spot. However, experience and experts have taught me there are solutions for writing successful letter and email openers.