Computed fields - is this really necessary? (And how to?)

I've been trying to follow this tutorial on computed fields https://www.drupal.org/node/2112677 and I don't get why computed fields are added in the `bix_profile_entity_base_field_info` hook? Is this really necessary to do it in the hook or when adding the field to my own entity I can do it in the entity's `baseFieldDefinitions()` method?

I have tried both ways though and neither seems to be working on Drupal 8.2.0. When I go to canonical link of my entity in admin that uses the default entity view builder, the filed just does not appear there.

I define it like this:

        $field = 'duration';
        $fields[$field] = BaseFieldDefinition::create('string')
            ->setLabel(t('Duration'))
            ->setComputed(true)
            ->setClass(DurationField::class)
            ->setDisplayOptions('view', [
                'type' => 'string',
                'weight' => ++$weight,
            ])
            ->setDisplayOptions('form', array(
                'type' => 'hidden',
                'weight' => ++$weight,
            ));

`getValue()` (nor `__get()`) on the `DurationField` class does not get ever called either.