database - What's the point of a candidate key?

Question

I'm fairly new to database management and this question never seems to be answered in more than one sentence. All other SO answers say "A candidate key is a minimal super key." That means nothing to me.

A candidate key is supposed to specify uniqueness of a db record, correct? And a primary key is a candidate key. If a primary key already specifies uniqueness, what's point of adding more candidate keys?

I have seen example records like the following:

Employee(ID, Name, PhoneNumber)

where ID is the primary key and PhoneNumber is a candidate key. From what I see, the ID is enough to specify the uniqueness of an employee record. Although PhoneNumbers are (probably) unique, specifying them as a candidate key does not seem "minimal" to me.

Answer 1

It means that if PhoneNumber was indeed a candidate key you could delete the ID column and use PhoneNumber instead. In other words, it is a candidate for being a unique key.

Wikipedia has a more formal definition that you many want to look at.