Get benefit of current BTree implemenation of indexes in Mongodb

gistfile1.txt

> db.test.find().count()
13
> db.test.find()
{ "_id" : "01-02" }
{ "_id" : "01-01" }
{ "_id" : "01-03" }
{ "_id" : "02-01" }
{ "_id" : "02-02" }
{ "_id" : "02-03" }
{ "_id" : "03-01" }
{ "_id" : "03-02" }
{ "_id" : "03-03" }
{ "_id" : "03-04" }
{ "_id" : "03-05" }
{ "_id" : "03-06" }
{ "_id" : "01-04" }
> db.test.find({_id: /^02-/}).count()
3
> db.test.find({_id: /^02-/}).explain()
{
	"cursor" : "BtreeCursor _id_ multi",
	"nscanned" : 4,
	"nscannedObjects" : 3,
	"n" : 3,
	"millis" : 0,
	"nYields" : 0,
	"nChunkSkips" : 0,
	"isMultiKey" : false,
	"indexOnly" : false,
	"indexBounds" : {
		"_id" : [
			[
				"02-",
				"02."
			],
			[
				/^02-/,
				/^02-/
			]
		]
	}
}