An applause light statement is designed to gain the support or agreement of an audience, the same way an actual applause light or sign indicates to the studio audience that this is the part where you should start clapping.
Usually there isn't much substance behind an applause light statement, and you can spot this by trying to reverse the statement. Consider someone saying "we need to ensure technology benefits everyone, not just a few people". If you reverse it, you get "we need to ensure technology benefits a few people, not everyone". Since the reversed statement sounds odd and abnormal, the original statement is probably conventional knowledge or the conventional perspective and doesn't contain new information. The statement isn't designed to deliver new information or substance, just to gain support.
Too many applause light statements is a characteristic of empty suit business talk, the "inspirational speaker" and the charismatic politician. Very little substance, merely an attempt at influencing perception. Sometimes, though, applause light statements can be useful, like when you want to establish common ground or set the stage for an argument. But most of the time, these statements are used merely as the "easy way" to write a speech and influence an audience.
And yes, applause light statements are more common in speech than in writing. In writing, the reader has more time to process what you said and call you out if you have zero substance behind your words. In speeches, you don't have this luxury, and it's pretty hard to hold a negative opinion of a speech if everyone else around you was nodding and applauding the entire time.
Eliezer Yudkowsky, who coined the term, wrote a tongue-in-cheek speech using only applause light statements. It looks eerily similar to just about every speech I've ever heard:
"I am here to propose to you today that we need to balance the risks and opportunities of advanced artificial intelligence. We should avoid the risks and, insofar as it is possible, realize the opportunities. We should not needlessly confront entirely unnecessary dangers. To achieve these goals, we must plan wisely and rationally. We should not act in fear and panic, or give in to technophobia; but neither should we act in blind enthusiasm. We should respect the interests of all parties with a stake in the Singularity. We must try to ensure that the benefits of advanced technologies accrue to as many individuals as possible, rather than being restricted to a few. We must try to avoid, as much as possible, violent conflicts using these technologies; and we must prevent massive destructive capability from falling into the hands of individuals. We should think through these issues before, not after, it is too late to do anything about them..."