Why do so many programs use relation databases instead of loading the data during startup and keeping it in memory? Especially for smaller datasets I would think, that a database adds unnecessary complexity and overhead. Also, a lot of data can be saved using modern RAM and when using an in-memory approach, optimized data structures can be utilised to further improve the performance
Edit: yes I meant relational databases
It all depends on the implementation and need.
In-memory structures are usually faster to work with, but harder to coordinate multiple updates from multiple sources (different applications, services, etc).
Databases have all sort of failsafe mechanisms to ensure data integrity and recovery options, in most times there is no need to reinvent it all over again.
Persistent - do you need to access the data again once your program was finished? How often does the data change by other programs/tasks once you read it? How big is your data and how complex are the connections between your data objects?
Many times the implementation is a mixed approach. It is better to know and calculate the needs before you start your project, but as it usually happen, once you get performance issues, you start optimizing adding in-memory cache or scale to a bigger database.