What data is typically stored on RFID cards and why?
in my experience from some extremely limited RFID security research is that most organisations implementing RFID cards are simply utilising the cards UID and the rest of the storage is blank.


You are right, this is typically how RFID cards like Mifare are used in simple access control schemes.

However, there are more complex schemes where being able to store data on the card can be beneficial. It is especially important in applications where:


The number of users is huge
The access rights are a bit more complex than just "allowed everywhere" / "disallowed"
You don't need to record each access in a central database (you just need to check the rights)


In this case, contacting a central backend for each access is not required, and would actually become a bottleneck. The data storage ability of the cards can be used to solve the problem much more efficiently, and you don't even need the access doors to be connected to a network anymore (which can also be beneficial in applications where network connection is not reliable enough, or not available at all).

There is one typical application where all these requirements are very important: public transport.

Checking rights against a central database for each traveller would be a huge problem. If you still want to track and record accesses, you can do it on a local database which make things much simpler. You can even simply record it on the card itself (another use of the card data storage ability): this way, you can easily tell your customers you're not spying on them (makes things much easier legally), but still have access to the history data if the customer is willing to provide his card.

As a practical example, here is the kind of information that are stored in Oyster cards (London public transport):


Generic Card Data: Identification number of the card, Pay As You Go 
(PAYG) Balance, Passenger type, type of discount, Photocard 
identification number if applicable, Staff identification number if 
applicable, the deposit value, and Registration flag. 
PAYG top up Data: Date, Time, Location, and value added. 
Ticket Data: Type of ticket, start and expiry date, and time 
restriction if applicable. 
Transaction Data: Date, Time, Station number or bus route, and fare 
charged.


(Source: https://www.whatdotheyknow.com/request/data_stored_on_oyster_cards)

Outside of public transport, you could imagine applications where some very personal data needs to be used to grant access, and you don't want this data to be stored in a database. Typical example is biometric data (e.g. fingerprint), for which there are specific legal considerations (you can't just store this anywhere you want, depending on the country).