Just as someone that works with robocalls, what usually happens is they have your number from either something you have been participating in (for example, a health study that you have participated in the past, we've sent you a form and not received it back, etc.) and the numbers get loaded in with your time zone based on your area code. Usually what happens is they call you at three different times during reasonable daytime/evening hours until they either get a pick up, or an answering machine. Our usual protocal is after three robo attempts we give up and either put you in tracing or enter something that stops the expectation in the system. As annoying as I know they can be from personal experience, they are a really economical way for people to either get reminders or respond to a survey if that's what the intent of the robocall is.
Sorry to everyone out there that rightfully finds them annoying, obviously some are not always for great reasons, but sometimes they are.