Proper tagging of a sentence can enhance the quality of downstream processing tasks. If we know that sue is a verb and not a noun, then this can assist in establishing the correct relationship between tokens. Determining the POS, phrases, clauses, and any relationship between them is called parsing. This is in contrast to tokenization, where we are only interested in identifying word elements and we are not concerned about their meaning.
POS tagging is used for many downstream processes, such as question analysis and analyzing the sentiment of text. Some social media sites are frequently interested in assessing the sentiment of their client's communication. Text indexing will frequently use POS data. Speech processing can use tags to help decide how to pronounce words.