Question about first proviso to section 147

This is what the section says. 

Provided that where an assessment under sub-section (3) of section 143 or this section has been made for the relevant assessment year, no action shall be taken under this section after the expiry of four years from the end of the relevant assessment year, unless any income chargeable to tax has escaped assessment for such assessment year by reason of the failure on the part of the assessee to make a return under section 139 or in response to a notice issued under sub-section (1) of section 142 or section 148

But since assessment under section 143(3) or Section 147 has already been completed once,  this means that return under section 139 or the return in response to notice under section 48 must already have been filed so why is this condition even mentioned in the proviso for enabling the AO to reopen assessment because it will obviously never be fulfilled.