I am to start workin on an inventory database design and my boss keeps telling me to use master(fixed) tables and transaction tables and if possible temporary tables

i know the role of master tables and also the definition a transaction table
but why is it necessarry
I will just append,update and delete directly from the master table isnt that easy?
Suppose i have 4 master tables then will i need to have 4 transaction tables with the same structure
Do advice ppl!
thnx