Line chart can show one or more measures as measures, such as Sales Amount, Total Costs, Quantity of the goods sold and etc.
Depends on the variety of values across a time period, Line chart can illustrate a straight line as a trend. This trend is good to understand how in overall products are selling, is revenue going up or down for example.
You might want to define minimum, maximum, average, or median values for your line chart as separate lines, and compare values lines with these lines. These are reference lines which can be dynamically created based on value of measures in the chart.
I think this was a rather helpful post in figuring out what Power BI is capable of doing, although if you build a lot of charts that look like Reza’s last example, you probably want to scale that back a bit.