It’s a way of explicitly remarking the free part. Before OSI’s definition Open Source referred to permissive licences. In most cases it still refers to permissive licences, thus the clear distinction is relevant.

Unless people starts to refer to BSD, Apache and similar as open source permissive in order to differenciate with open source copyleft (or similar).

Otherwise I feel is completely relevant to refer to copyleft software as Free Software. It helps both to show that there’s differences between both and also makes new people realize that there are different alternatives.