I think trendlines are a great way of identifying the market direction. Understanding the direction of an underlying trend is one of the most basic ways to increase the probability of making a successful trade because it ensures that the general market forces are working in your favor. Many technical traders draw trendlines to identify an uptrend or a downtrend and will use this tool as a guide for when to sell as it can also be an early indication of a trend reversal. The break below an uptrend line is in itself a sell signal, attracting new sellers to the market.
If you want to learn more about trenlines check this blog:

Trendlines Potential CharlesMcG’s Blog