Trend lines are drawn on charts to help predict the general direction of price.It also help to determine good entry and exit point.they also help to see the reversals.
of course, personally I think it is true that the line that is drawn over pivot highs or under pivot lows to show the prevailing direction of price. Trendlines are a visual representation of support and resistance it gives you an idea of the approximate future actions of a trending currency pair. Trendlines are a basic part of technical analysis.