"How can we use light?" is a question. This is asking for the ways that light can be used.
"How we can use light." is a title of an article, most likely. The article would discuss how light can be used. As a title, it would look like: "How We Can Use Light"
"Why do we need to talk?" is a question. This is asking for the reasons why they need to talk.
"Why we need to talk." is probably the title of an article. It would list reasons why they need to talk. As a title, it would look like: "Why We Need to Talk"
The non-question phrases are not complete sentences, either. They cannot stand alone. This is why I believe they are titles.