Why is a post's subject so important?

The first post of a given topic establishes the subject by which all subsequent replies will be known. Once there's been a reply, the topic subject cannot be subsequently changed. It's therefore important to get the subject right from the outset. Make it as descriptive and as specific as possible.

For example, "How do I change batteries on a Ninja LED Poi?

is much better than something completely generic, such as "Newbie needs help!". Not only is it more likely to elicit a response, but it'll also make it much easier for all posts in the topic to be subsequently located.