Tables are useful for demonstrating patterns, exceptions, differences and other relationships.
Everything on a table should be labeled - rows and columns should have units of measurement, and totals. All codes, abbreviations, symbols, exclusions, data sources should be explained in a footnote. If a table is set up right the data can speak for themselves. If the table is part of a report it should still make sense by itself even if it got separated from the report. You can always describe data with a narrative, but you would be expecting a lot from your audience to digest numbers and words, when a table can summarize data more succinctly.