why is it necssary to use comparative balance sheets, a current income
statement, and certain transaction data in preparing a statement of
cash flows?

The purpose of the statement of cash flows is to identify the primary
activities of a fiscal period that resulted in cash inflows and
outflows for a company.  The statement describes the cash flow results
of financing, investing, and operating activities for a company for a
fiscal period, and it explains the change in a company's cash balance
during the period.

Operating activities are transactions involving the acquisition of
production of goods and services and the sale and distribution of
these goods and services to customers.  An important relationship
exists between the income statement and the operating activities
section of the statement of cash flows--both are based on the same set
of activities.  An important relationship also exists between the
comparative balance sheet and the statement of cash flows--when using
the indirect method the decreases and increases in the accounts can be
measured on the comparative balance sheet.

I hope this helps!

JayNick