Stretch is how far the garment can give beyond its stated measurement on your size chart, as a percentage of that measurement. It decides which sizes survive rather than which one wins.
Why stretch at zero causes problems
With stretch at zero, any body larger than the garment measurement is ruled out immediately. That is correct for a stable woven shirt, but wrong for:
Jersey
Ribbed knit
Swimwear
Activewear
Anything with elastane
Zero stretch is the most common reason a size chart stops returning results for larger shoppers. Set stretch to a realistic percentage for the fabric to allow those sizes back in.
What stretch will not do
Raising stretch does not shift the recommended size among sizes that already fit. If you want a different size recommendation, adjust ease instead.
A useful side effect
Raising stretch can turn a no-result into a size recommendation with an honest 'this will be tight' warning. Most shoppers prefer that to a dead end.