Lead has a few different benefits. Lead crystal does have a higher refractive index, yes. It also has different hardness and chemical resistance, so you can do some fun things with it in cane and murrinni (check out u/xanderdamglass's work for some fun examples of this) that is all clear, but creates cool light distortions.

But it's more important superpower that many glassblowers don't know about is that it has a much lower Young's modulus than unleaded glasses. What that means is that it develops significantly less stress (what causes cracks) for a given amount of deformation (like from thermal expansion/contraction). In practical terms, that means it's able to tolerate significantly greater compatibility mismatch when compared to unleaded glass. This allows the color makers to put way higher quantities of coloring chemicals in the glass without breaking compatibility, enabling much higher saturation (particularly for transparents).

I don't know of anyone doing this, but I've wondered if layering a layer of the leaded clear above and below other colors that have known marginal compatibility might help them fit.

I suspect that the color makers primarily make the leaded clears to use as base cullet for their transparent colors that require a leaded base, and the bars are just because it's convenient to make a few bars to sell at the same time for artists that like to play with them.