Yoast answers this very well in this blog post:
A better solution would be to add a
<meta name="robots" content="noindex, follow"> tag to those search results pages, as it would prevent the search results from rankings but would allow the link “juice” to flow through to the returned posts and pages.
Someone will inevitably link to a page you wish Google to not index. However, if you nofollow the links you are losing the benefit of that link. That page likely has links to your home page and other pages on the site which aren't noindex.
In certain cases you may have some pages you do want indexed but are only linked from noindex pages. (Although I would argue that in this case the site structure probably needs improvement.)