What is so important about the state of West Virginia? What would happen if it did not exist? why is it valuable?

Many have argued that the Civil War in these United States never answered the question of state secession, in other words, if states could voluntarily leave the Union.  The Constitution in Article IV Section 3 states how states can join the United States, or how states may be re-formed, but nowhere does it state how a state may leave the United States. By virtue of the war, the Confederate States did in fact leave the United States, only to become reestablished after the war.  When secession came, Virginia could hardly complain that its western counties were leaving; Virginia seceded, then West Virginia seceded from Virginia!  West Virgina, by its very existence, argues that at least sections of states can leave states, if not states leaving the Union.  However, a careful reading of the statute indicates that the Secession of West Virginia and its admittance into the Union was technically illegal by Constitutional standards, as Virginia never consented to it becoming a separate state (clearly the Founders never considered the case of part of a state actually becoming part of a whole other country.)