They're not just for high-rpm engines. They're for those situations when an off-the-shelf cast intake just isn't right for the combination and can't be modified to be within reason. Sheet metal intakes can be made in any variety you can imagine. They can select and build them with the exact runner length, taper, CSA, plenum volume, etc. as needed. And there are many heads that require a fabricated intake since cast ones are not made. If you buy a used one you need to be very careful to make sure it is EXACTLY what your combo needs for each of the parameters I mentioned above. I lucked out finding a very nice used one for my 331". I got over $3200 worth of intake and tops/spacers for far less than that.