While it's not always the case, the most common reason is that markers represents the data that has been entered/registered, while the lines represent the trend or "traveling" within an X period of time where data goes from
N to N', N'' and so on.
Edit: see example below:
20, yet it's impossible to tell if there was data for
Feb or if the line passed thru
Feb by going
from ene to mar. Or in other words: We want to visualize data, yet we can't give proper information and leave data as ambiguous (which is the exact opposite of what we should do)
Image Source: Uxpanol.com
When you have peeks and valleys, we can assume the vertex is a data point. However, let's assume data goes always in more or less the same direction. For example, it goes always up. Without the data points, you won't be able to visualize whether the specific axis has data or not. Just imagine we have
day1, day2, day3, day4 on the X axis. Then
100, 200, 300, 400 on the Y axis. Without data points, you could have 2 scenarios. We have data for all 4 days, or 3 days, or just day1 and day 2 going from 100 to 400, with day 2 and 3 registering no data. You can see this woudn't make any sense
And that's why you add data points: for increased visualization.