It makes you able to do e.g.:
scala> def foo(as: Int*)(bs: Int*)(cs: Int*) = as.sum * bs.sum * cs.sum
foo: (as: Int*)(bs: Int*)(cs: Int*)Int
scala> foo(1, 2, 3)(4, 5, 6, 7, 9)(10, 11)
res7: Int = 3906
It makes you able to do e.g.:
scala> def foo(as: Int*)(bs: Int*)(cs: Int*) = as.sum * bs.sum * cs.sum
foo: (as: Int*)(bs: Int*)(cs: Int*)Int
scala> foo(1, 2, 3)(4, 5, 6, 7, 9)(10, 11)
res7: Int = 3906