There are a lot of skills and tools that seem a little out of place in our practice and training material. There is always a ‘why’. PERT, Program Evaluation and Review Technique is one of those.
The PMBOK states PERT (or the 3-point estimate) is useful when you don’t have historical information to rely on. This is a good start to understanding.
PERT is also a way to determine a range and probability estimate using the Central Limit Theorem and Normal Distribution. It gives you and your stakeholders a better expectation of not only an estimate but a confidence in that estimate, plus its range. Confidence considers risk or probability and range determines how closely your estimates are to each other (‘variance’ in your estimates).
All this communicates more effectively than just providing a single point.