Retrospective cohort designs can yield some measures of incidence in patient populations. However, researchers are limited to the variables that have been collected in an objective fashion within homogeneous populations. Incidence is a much more valid measure when generated using a prospective cohort design. Researchers choose in an a priori fashion exactly what variables will be collected in the measured.
Incidence is a much more precise measure of association versus prevalence. Prospective and experimental designs can yield measures of incidence and establish the relative risk of developing an outcome. Researchers and clinicians also have a better understanding of incidence and relative risk versus prevalence and odds ratios.
Longitudinal data is data collected over an extended period of time. Longitudinal data is necessary for understanding the etiology and progression of disease states. Survival and time-to-event analyses produce popular measures in medicine such as 1-year, 3-year, and 5-year survival and recurrence. The primary issue with collecting longitudinal data is attrition and loss to follow-up with the prospective sample. As participants fall out of the study or are lost, the validity of the data greatly decreases.
Again, it is important to state that prospective designs give more control to researchers in regards to what data is collected. Every variable that you find pertinent for establishing causal effects between predictor and outcome variables, when controlling for all important demographic, prognostic, clinical, and confounding variables, can be chosen and collected. Observational biases associated with retrospective research do not apply with these studies because you can collect all of the data on all the variables that you chose, given that there is a theoretical, conceptual, or physiological reason for doing so.