Question: I Am trying to learn the RDBMS. I have question for you guys. Why does a DBMS interleave the actions of the different transactions instead of executing transactions one after another?

Answer: 
It is to increase the transaction throughput. Assume that there are five billing machines in a super market and all these machines are connected to a database server. If the server handles transactions in serial order, ie., one after the other, then the use of 5 billing machines is a mere waste. If many transactions are able to be executed simultaneously by the server, then we could add even more billing machines and we are able to generate many bills.

For more information on [transaction management and concepts please visit here](http://www.exploredatabase.com/2016/04/transaction-management-in-database-management-systems.html).
The transaction can be defined as the group of tasks or the program unit which after execution changes the contents of a database.
To decrease waiting time of users DBMS uses concurrent transactions.
[DBMS Transactions](https://whatisdbms.com/explain-dbms-transactions/)