We don't have to store all properties in database such properties are called transient, they are useful as they show current behavior of user profile and Profile controls what is user seeing in the web sites like promotion or targeter result. Yes we can we see transient property in ACC or BCC.
Consider a scenario when An application contain different module like login, Servcie Layer, ATG. All three module work together and provide service to user. login module handle its data and save using some other technology and It is expected that ATG has to take care of login property also while displaying promotions. In this case ATG will make some transient property which is going to be updated while login take place and when user surf through the sites promotions are being dispalyed on basis of user all properties.
Hope this help.
As explained, transient properties are run-time properties which are not necessarily be saved in DB. since, data is not persisted transient properties helps in increasing the performance during repository process.
Suppose, we have a process where user is trying to place an order by providing their credit card info. Since, most of the payment process is handled by external payment gateways (due to lots of business policy) it is often recommended that the customer's personal data (PII) should not be stored in internal database. But to call several external services and post processing requires PII data to be retained in the repository item. It is here transient properties helps to retain the run-time values for complete transaction processing.
Hope it helps. Correct me if I'm wrong.
Here's a very simple example that'll answer your query:
Suppose you have an album that has n number of songs in it. Now when you display the album details on your application you need to show the album lenght. So album lenght can be used as a transient property. You need not save this part of information. So you can create this transient property and write the property descriptors to calculate the total lenght of the album by adding the lenghts of the individual songs.
Hope this is clear.