The short answer
Sentiment analysis is the process of classifying mentions of a brand, product, or topic as positive, negative, or neutral, typically at a large enough scale that reading every mention individually would not be practical. It gives a broad-strokes sense of how something is being perceived across a large volume of mentions.
What it is genuinely useful for
Tracking sentiment over time can reveal a real shift. A sudden spike in negative sentiment around a launch or announcement is a signal worth investigating, even before reading every individual comment behind it.
Where it falls short
Automated sentiment classification struggles with sarcasm, industry-specific language, and genuinely mixed opinions, and can misclassify a fair amount of nuanced feedback. It is a useful signal for spotting a broad trend, not a substitute for reading a meaningful sample of the actual comments behind a sentiment shift.