Google recommends putting the Nofollow attribute on links whose quality you cannot guarantee.
For example, a space without moderation when it is possible to post links.
According to Google, implementing the nofollow attribute on a link today indicates that you should consider it as a ‘hint’. However, a link in nofollow seem to be followed by GoogleBot.
Still according to Google, a link in nofollow will be taken into account in the calculation of the popularity distribution sent from a given web page. Using the nofollow attribute on an internal link would therefore be a waste of popularity.
All this remains to be proven and tested.