Definition of Previous Year

As we all know that "Previous Year" means the period which begins with the date immediately following the last day of the previous year relevant to the assessment year commencing on the 1st day of April,and ends on the 31st day of March,But in the case of newly setup business coming into existence on or after the 1st day of April"Previous year" in relation to the assessment year commencing on the 1st day of April, shall be the period beginning with the date of setting up of the business or profession or, as the case may be, the date on which the source of income newly comes into existence and ending on the 31st day of March, whats the logic behind it? In this situation how can we say the Previous "year".