Other than reuse, is there any reason why I should not link directly to a form in Drupal 8? I mean linking directly to the webform rather than to a node that incorporates the webform?
The main reason being you can't use any functionality that comes with a node being the page. E.g. add custom fields/paragraphs and allow users to edit them to customise the page. All of that would need to be done as blocks which isn't as efficent/easy for content loaders to manage
If you need to add metatags and other things like that also become more difficult.
And from the webform perspective it allows you to reuse the same form on multiple pages and the results are segmented to the webform node. Meaning if you need multiple copies of a form on your site you can have 1 form and reuse it on multiple pages rather than duplicating the actual webform itself