Vacuum leaks are a common problem and have been since time immemorial on cars. Sometimes a hose works loose, or it ages and cracks, or a check valve is present that sticks. Sometimes a hose gets blocked with crud and then vacuum will not get where needed.

With modern fuel systems that derive the amount of fuel injected from some measure of the intake air flow, air that gets in via an abnormal route is not going to be measured by the air flow sensor(s). Then the air/fuel mixture will not be right. If bad enough the engine managment system will not be able to compensate and some symptoms will develop and/or you will see a malfunction (check engine) light.

A visual inspection under the hood once in a while can be a good preventive measure.