msmshazan
What is the use of having SQL and DBMS and all those stuff which I don't understand?
Well I can't comment on the other stuff but what I can tell you is that database systems (namely Relational/SQL DBs) are built for
Handling large sets of data. So, the DB system will be quicker at processing huge data sets, such as 25,000 and up, than using a linear standard type processing in your applications. For this reason, people choose to use these databases, which use a different logic that is based off of set theory. In the end, the databases themselves are still programs written in C and stuff, but the idea is that they are optimized for handling large sets of data at once versus acting on single pieces of data at a time such as in a for loop.
As Jesse said, try to refrain from blanket statements such as "All xyz MUST ALWAYS do/have abc" because there are always exceptions in life. However, databases are certainly a popular choice in financial applications, amongst many other types. This forum we're looking at right now has stored this very post in a database.
If anyone here is interested in writing some database software in C, let me know.