Depreciation is the estimated wear & tear in a company's tangible assets during a certain period of time. It is also deducted from revenues in calculation of net profit which shows how much value the company has derived in net on an accrual basis during the same period from all activities.
To get how much value the company has derived only through operating activities instead on a cash basis in the same period (which is operating cash flow), non-cash expenses such as depreciation and non-operating expenses and income, respectively, such as interest expenses and income, all of which are included in the income statement, and changes in operations-related balance sheet items such as receivables, inventory and payables are, as per necessary, added to or subtracted from net profit in the calculation via the indirect method.
Investing cash flow measures in general how much the company has spent on acquisition and disposal of long-term assets on a cash basis. Adding depreciation to cash spent and received in acquisition and disposal of long-term assets, respectively, would yield a misleading result.