Those particular rocks seem to be sandstone, though... which would pretty much be proof of liquid water having existed at some point.
Rocks are rocks, sure... but rocks tell stories, and these ones are telling a story that, while common and somewhat uninteresting on Earth, is quite interesting indeed when told on Mars.
You are totally right. I was going to make a big post about how they actually formed but I deleted it halfway through. These rocks are definitely significant in that the same processes on earth happen on Mars so we can infer how they were deposited and formed. Hence my shitty first comment, rocks are rocks.