A data base management system (DBMS), such as MySQL, PostgreSQL, Oracle, and others, gives you a convenient and efficient way to store, search, modify and retrieve information from one or more tables in one or more data bases. The DBMS' are relational which means that you can store related information in multiple tables (rather than one big file) and retrieve it quickly (an example may be a customer table storing information about a customer and an orders table storing information about items a customer has ordered and a products table storing information about the products. Say the customer - orders table is joined (related) by, say, customer number and the orders - products table is joined (related) by product number. For example, can then find out how many customers ordered a given product quickly (handy for inventory control purposes, say) rather than searching through massive text files looking for information. Without a DBMS it would be virtually impossible for, say, Amazon.com to exist.
So, is there anything wrong with searching giant text file? Nope, but it sure makes sense to do so in the most efficient way available. A DBMS keeps duplication down (say you sold 1,000 of a given product and had 1,000 identical lines of text describing the product in a file rather than just one description that is found by a number and displayed, printed or whatever needs to be done.
Hope this helps some.