Just from a social perspective, people need to know them to value them. With so many people living in urban areas these days, if we want natural spaces to be valued, then people have to be able to get to them and they have to be there for them in the first place. That's a pretty obvious reason.
I think the other thing, though, is that.... I'm not an expert on this and I've probably done the same reading as everybody who's here, but it seems to be pretty clear from the science that there are pretty huge social benefits of people having access to natural spaces, in terms of their health, wellness, crime levels, and everything like that. When you want to talk about livable cities, I think it is a very important aspect, and this is not to take away from the protection of ecosystem integrity elsewhere.