I want to add to other answers that setters can be used to prevent putting the object in an invalid state.

For instance let's suppose that I've to set a TaxId, modelled as a String. The first version of the setter can be as follows:

private String taxId;

public void setTaxId(String taxId) {
    this.taxId = taxId;
}

However we'd better prevent the use to set the object with an invalid taxId, so we can introduce a check:

private String taxId;

public void setTaxId(String taxId) throws IllegalArgumentException {
    if (isTaxIdValid(taxId)) {
        throw new IllegalArgumentException("Tax Id '" + taxId + "' is invalid");
    }
    this.taxId = taxId;
}

The next step, to improve the modularity of the program, is to make the TaxId itself as an Object, able to check itself.

private final TaxId taxId = new TaxId()

public void setTaxId(String taxIdString) throws IllegalArgumentException {
    taxId.set(taxIdString); //will throw exception if not valid
}

Similarly for the getter, what if we don't have a value yet? Maybe we want to have a different path, we could say:

public String getTaxId() throws IllegalStateException {
    return taxId.get(); //will throw exception if not set
}