The Lead Block is a block that is a bulk storage for your Lead Ingot.
It can also be used as an attractive and unusual metallic looking decorative block.
Compatibility with other mods
If you have another mod installed with a different variety of lead, normally you can mix and match the ingots to craft a Lead Block.
If you have another mod with a different type of Lead Ingot, a Galacticraft Lead Block will be made from mixed lead ingots so long as you use at least 1 Galacticraft Lead Ingot in the recipe. If you use zero Galacticraft ingots then a Galacticraft block will not be made. This requirement for minimum 1 Galacticraft ingot in the Galacticraft recipe allows you to control whether it is the Lead Block from Galacticraft which is made or the lead block from the other mod.