Why do we add back non cash items to net profit while calculating cash flow from operating activities?

The Profit--both the Gross and Net--shows the Difference between the Cost of Sales and Sales Achieved in Monetary Terms during a particular period.  As such, it shows the Additional Cash  raised by the Organization, over and above the total costs incurred .  However, in the Cost of Goods Sold, certain items of Notional Expenses(non-cash expenses)--Depreciation-- etc. are also included, in order to have a near actual expenses incurred.  Thus, the Profit would be showing a lesser figure than the actual. 

The Cash-flow, as the name itself suggests, shows the Total Cash generated by the business during a particular period.  As such, the Profit, which is the prime source of cash generated, is re-adjusted for the notional expences included therein.  Hence the non-cash items are added back to the Profit, which calculating cash-flow from operating activities.

 

When we prepare cash flow statement, we only account for items involving cash flow either inward or outward.
Hence all non cash items are added back that were deducted in the computation of net profit.

Adding back non cash items to NP because that does NOT have/had any impact on Physical or REAL CASH movement of the company.

When measuring the cash flow in and out of a business, it should reflects how much cash is generated from each activity.
specially the operating activities.
as you mentioned its non-cash transactions so we should elemenate it's effect in order not overstate or understate the net cash generated or used in operating activities. 

becuase the depreciation not accual expences so it s added  back  to the net brofit  to find the accual cash that generated in the business

When we prepare cash flow statement, we only account for items involving cash flow either inward or outward.
Hence all non cash items are added back that were deducted in the computation of net profit.

Because D&A are non cash expenses and thus must be added back to calculate the cash flow

the word/term itself will explain on its own.
In order to know the "Cash flow from operating activities (note the word cash flow from) the non cash items are added back.
The non cash items will not give any impact in cash flow.
The net profit is calculated after including provisions etc for which cash is actually not paid.

Cash Flow Statement as by name is the statement to show the flow of cash only. Net Profit has effects of non cash items e.g. depreciation, therefore it's necessary to add back such items to get the real amount of cash actually moved.