Many events that researchers want to study are rarely visible – because they’re simply rare, or because they’re normally kept hidden. Indirect observation allows you observe something that’s caused by what you want to study, as opposed to observing directly whatever you want to study.
An example is a study of premarital sexual behaviour in the 16th century. Direct observation wasn’t an option to the 20th century researchers who conducted that study, but they did have access to 16th century records of marriages and of births. They also had access to church records. So by indirect observation, they were able to conclude that in the community they studied, an unmarried woman of childbearing age was more likely to be sexually active than to be a regular churchgoer.
Most indirect observation is more mundane; it often involves activities such as looking at patterns of wear on objects; that can tell you a surprising amount about its use. Indirect observation is a handy approach to have in your toolbox.