What are some reasons to apply linestyles and markers on a graph?
Generally, linestyles and markers are applied to make the lines on a graph easier to understand, and to mark specific points on the graph clearly.
For example, say that we wanted to see the growth of a stock price over time, but also wanted to see its value for the first day of each month clearly. By using markers on a line, we can see both the progression of the price over time, and also see the precise points for the prices each first day of each month. In a way, the line part lets us see the overall progression of x vs y, while markers let us see precise values at specific points, like checkpoints.
Linestyles and markers are especially helpful to apply when you start to have many lines plotted on the same graph, because it may become harder to distinguish them.
Furthermore, line styles and markers are not absolutely necessary all the time, but including them can make a graph easier to understand and give it a more professional feeling. Do what works for you!