Tickers are exchange-specific and reusable. The same letters can mean different companies in different countries, and a symbol freed by a delisting can be handed to someone else later. That is why back-testing and record keeping should use a cusip or isin rather than a ticker.
Symbols also change with corporate events: a merger, a reverse-merger, a rebrand, a reverse-split flag, or bankruptcy status via a bankruptcy-ticker-suffix. Class structures get suffixes too, so one company can have several tickers.
Example: a database keyed on a ticker shows a smooth ten-year price history, but the symbol belonged to a company that was delisted in year four and reassigned in year six. The chart is two unrelated businesses stitched together.
Related: cusip, isin, share-class, bankruptcy-ticker-suffix, delisting