nofollow is a link relationship value used to qualify a link; Google treats it as a hint rather than a guarantee that the destination will not be crawled or indexed.
It matters because technical issues can prevent great content from being discovered, rendered, indexed, or trusted. Strong technical foundations protect rankings during redesigns, migrations, content expansion, and platform changes.
Example
A site may mark a link with rel=nofollow when it does not want to provide an ordinary endorsement. Google treats this as a hint; it is not a reliable way to prevent the destination from being discovered or indexed.