First post - had to comment

In your question you must think of two critical ideas:
1. for FCFF, you are trying to determine what capital is FREE to distribute to both types of “owners” of the companies assets. The owners are either the lenders or the shareholders. For FCFF, you are trying to determine the free cash that is available to either pay back lenders in either principle or interest, or pay shareholders. The term free here is key, because you can consider it the cash that is available after you have re-invested in both fixed assets and working capital in order to sustain operations.
2. You must understand how to arrive at FCFF from different points in the financial statements. In your question, you are started at Net Income.
The process is then simple for why you add back the aftertax interest, but not depreciation.
Starting at Net Income of 1000
Depreciation is 200, Investment in Fixed Assets is 100, Change in Net working capital is 50.
To get FCFF, we need to first arrive at operating cash flow (Net income + depreciation) = 1000 + 200
1200 = operating cash flow, which still includes interest. Lets say we don’t adjust for interest first and calculate FCF = 1200 - fixed assets (100) - change in NWC (50) = 1050
We now have FCF, but the problem is that this is not considered FCFF, because this is free cash flow after we have still paid the interest to our lenders. This is not Free cash flow before we paid all of the owners of the companies assets in some form.
So to adjust for this, we need to add back the interest. However, if we didn’t pay the interest at all, the company would not have also received the tax shelter from the interest payment. Therefore we need to add back the after-tax interest payment to get to FCFF, free cash flow before paying out in any form to any “owner” of the companies assets.
So now we can add the after-tax interest add-back to our formula and get
Net income + depreciation (operating cash flow) - investment in fixed assets - changein net working capital + after tax interest payments to lenders.
Hope this makes more sense.