The pendulum can be used for a multitude of purposes and questions. It's one of the reasons it is such a popular tool for Witches as well as others. It can be used for:
Making a choice of any kind.
Any question that involves a yes/no (or maybe) answer.
Figuring out what is best for you to do.
Finding out how you really feel about something.
Finding missing objects.
Finding missing pets.
Finding water, or oil, or gold etc.
Determining someone's compatibility with you.
Picking lottery numbers (you never know!).
Asking questions of spirit.
Almost any question you can think of can be answered in its most basic form using a pendulum. The list of questions you could ask is practically endless. The only limit will be a self-imposed one! The pendulum is not going to give you a complex answer to a question but anything that is capable of being answered with a "yes" or a "no" can be tried. If you do have a question that is answered by the pendulum but you definitely need more information than a "yes" or a "no" you can always consider a tarot reading as an alternative.
Remember though, the pendulum is not designed to take the place of your own common sense. When you first start using one it is tempting to consult it for anything and everything. That's fine for a little while as the practice will help you get the most from it. It is not intended to be an emotional or spiritual crutch, so if you find yourself always consulting it for everything, take a time out. It's a useful tool so don't waste its abilities.
Whenever you're going to have a session with the pendulum remember to recalibrate it first. Yes, it takes a few minutes and probably nothing will have changed, but do so anyway. It can save you from expensive or silly mistakes.