Edgar F. "Ted" Codd created 12 rules (revised with a 13th rule) to clarify the Relational Database Management System (RDBMS).
https://en.wikipedia.org/wiki/Edgar_F._Codd
Breaking any of these rules weakens data integrity and the ability of the system to manage the data. An RDBMS is not merely a bucket of data, but an entire eco-system for the management of data and data related activities. Before Codd's work, databases were managed on an ad-hoc basis as collections of files with links. It was a mess. Data was lost. Only the DBA knew how to find the data, and access methods could be very different for data in different locations. Accessing data could also be extremely slow. In addition to assuring the integrity of data, as well as managing it, relational database systems are very fast.
https://en.wikipedia.org/wiki/Codd%27s_12_rules
The "R" in RDBMS is "relational" and Codd invented the relational model of data. Key to relational data modeling is "normal form".
https://en.wikipedia.org/wiki/Database_normalization
The RDBMS world generally uses third normal form. Lower levels of normalization create additional work for data operations. Higher forms rarely show any improvements. The key concept of normalization is that a datum only exists in one place. In the RDBMS world where SQL implements relational algebra, normal form is both convenient and natural. In other venues such as paper ledgers, data stored in flat files, or in spreadsheets, normal form can seem awkward.