This improvement allows your text to be horizontally stretched or condensed by a percentage factor. It's often handy for tweaking the widths of fonts to fit a bit of extra text into labels (when used sparingly, that is... you can certainly abuse font stretching with some horrendous results!)
Note: This improvement requires Qt 6.3+ or KDE's 5.15 fork. (On other builds the stretch options are hidden and can't be set)