It is a pretty stupid concept, but mostly invented for the purpose of winning, rather than any ideological purpose.
Apparently when they switched to a more democratic system, the people elected hardcore liberals that were "unelectable" at the time... so they overreacted and made up a class of safe-guards.

Personally I'd like to see more liberals elected. But the Crats have been defeatist for a while, supporting their opponents' framing the word "liberal" as bad. Clinton is an embodiment of that.
The Democrats seem hellbent on making their political weakness permanent.

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"It's never over... BOY!"
The Tall Man, Phantasm III