Traders use chart patterns to anticipate where prices are heading, to set up targets, predict retreats and pullbacks, trend reversals etc. The tools the technical analysis provides are used mostly to determine key price levels for trades: at which price open (entry), the target (limit) and where to close a position if the price goes the other direction (stop loss).
Continuation and reversal patterns are the most powerful. It is important to take into account that an intraday trader will likely look for short-term patterns in timeframes of 1-minute up to 4-hours, while a weekly trader would likely use daily to monthly timeframes.