I think posters still have an important value, although today almost everything turns around digital marketing. The key point is that they work differently and should not be compared directly with the online. While digital advertising allows you to segment and measure results with great precision, posters play more in the field of local visibility and immediate impact.
For example, a neighborhood business, a gym or a cafeteria can gain a lot of notoriety by placing posters in strategic places with a lot of transit, such as groups of groups, universities or community centers. There you are not looking for clicks or digital metrics, but generate brand remembrance and capture the attention of people who move in your area of influence. In those cases, a good design with a clear message and a call to concrete action (such as a discount, a QR or an invitation to visit the premises) can be very effective.