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: 
A DBMS is typically shared among many users. Transactions from these users
can be interleaved to improve the execution time of users’ queries. By interleaving
queries, users do not have to wait for other user’s transactions to complete
fully before their own transaction begins. Without interleaving, if user A begins
a transaction that will take 10 seconds to complete, and user B wants to begin
a transaction, user B would have to wait an additional 10 seconds for user
A’s transaction to complete before the database would begin processing user B’s
request.
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).