Specular value controls the falloff. Default 0.5 gives about 1.45 or 1.5 (can’t remember which), which is close enough for most of the dielectrics. Use higher for more brilliant surfaces (glass or high IOR epoxy coat) and less for less reflective materials (like water). It’s a direct relation to IOR, but in more artistic friendly terms (texture and value wise). Manual has a formula that shows the relation, but a 0-1 value is much more convenient to preview and texture.
The problem of using layer weight, is that you can’t create realistic transmissive materials such as glass with it as the IOR needs to be inverted for backfacing faces (when you have an actual volume). For thin glass or translucent materials like leaves, layer weight would be a blessing as you don’t have to consider the backfacing faces.
For dielectrics, the facing curve should be 4% at facing angles and 100% at glazing angles.
A confusing aspect is that layer weight/fresnel, which uses 0-1 values, is not scaled the same way as the principled specular value. Not sure if it’s supposed to be backward compatible or just forgotten.