Dear for every report, there is a pre-defined period set defined by Tally to meet the general requirements after a complete market feedback. E.g. if we take a Ledger report, it will be shown for current month automatically as normally you would like to see only the current month's status and not a complete year's transactions. But take TDS reports, the date range is set automatically from 1.4... to current date as for TDS the whole years transaction need to be accounted for. same is for Profit and Loss account, it's automatically set to whole of the financial year till date.
When you open a company you don't need to select period everytime as the current financial year is selected automatically based on the last date of entry. Thus the period is automatically set at 1.4. ... to 31.3. ... and current date is automatically set to date of last entry.
In case some specific requirement is there, through customization you can pre-define date range based on report to report basis.
if u change d period at home page of tally u dnt need to change period again and again