It may be come from a specific requirement of company standards if there is, not industry codes or standards. Even if the gap must be helpful for welding (i.e. penetration, thermal expansion, etc.), the compliance with mechanical dimension and tolerances (i.e. ASME B16/31/36 series) and uniform gap may not be easy.
Please find the dimension in ASME B16/31/36 series, such as slip-on flange ID, pipe OD, and their tolerances (machining & fabrication). You can find the gaps are very narrow.

I believe the requirements may not be applicable to common weld joints other than specific weld joints (i.e. specific dissimilar joints, full penetration at fillet joint, etc.).