Why Pega have to introduce new option called ‘Configurable Sets’ especially for business administrators (application owners) ?
First of all, lets understand what is configurable sets? Its simple, its a variable/property. When you have a group of related data variables, its called as configurable set!! For example, loan limit, loan offer & other loan related attributes, can be called as a loan limits as shown below.

When there are many delegatable rules? Why Pega have to introduce this feature to update values at runtime?
We need to have clear understanding between delegatable business logic vs configurable business fields. If you have approval matrix defined in a decision table then its make sense for us to delegate that when compared to define that in configurable sets.
As per my understanding, now days buisness looks for quicker ways to update logics/fields/etc. So this makes a good point to have this feature from Pega Out of the box instead of some routine dynamic workaround(s)
Where to use Configurable Sets and How to create them?
You can refer Pega’s PDN article to understand more but simply to answer that, we can user App Studio to define these configurable sets.

Once you define, these appear as ‘Configuration Set’ in defining WHEN conditions as below.

We have DSS (Dynamic System Settings) variables right? Are they not same from conceptual point of view?
Absolutely Yes, both are same from conceptual point of view but not from functional point of view. Dynamic System Settings can be considered as design time settings before you run your application when compared to configurable set which should be considered as a runtime variable.
To update DSS variable, you must need Dev Studio to perform but to update your business field at runtime you need a simple portal for business admin(application owner) to perform as below.

Which are types[Delegatable Rules/DSS/Configuration Sets] should be in which are scenarios?
As per my understanding, you should consider DSS only when there are one time settings for your application to run where there is no need of business admins intervention.
As you all know we consider delegatable rules to delegate conditional business logics to the end users.
As explained in the above questions, wherever there is a need of simple business logic change at application or case level by application owner then you can consider this Configurable Sets.


