Well yes, what else could it possibly do? It’s just the way I understood you is that it’s configured to 2 coords to make lines by default and you don’t need to define an index buffer at all.
Ah, no… it’s configured to whatever the mesh mode requires… which is different for every mode, basically.
Yes, for example for a “line strip” you provide two vertices for the first segment and then one more vertex for each following segment. And for a “triangle strip” you provide three vertices for the first triangle and one more vertex for each following triangle. Used that back in the old days (2005 or 2006) when still many people coded like that in OpenGL. 
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