It can infer lots of stuff yes. But sometimes you have to write that part in a somewhat specific way to get synthesis to infer, or you might want the extra cont

It can infer lots of stuff yes. But sometimes you have to write that part in a somewhat specific way to get synthesis to infer, or you might want the extra control options that the inputs & outputs of a hard block instance offer, or you might have a more elaborate interconnection between several hard blocks and it ends up being easier to just instantiate these hard blocks and set them up manually.