Hi Chris
I think there are two scenarios here.
If you have a site and you want a subsite on a different WordPress install, there will be no impact on any other site. The plugin works within your WordPress install.
If you are importing to the same URL, within the same WordPress install then the plugin won’t delete content that you already have on the site, it will just overwrite with what is in your backup. If you only want the content of your backup to be present, you should start with a clean WordPress install. For example if you have a site with 20 images and you import a site with 1 image, and that image different, the imported site will have 21 images. Only one will be visible on the site as the site structure imported will only reference the imported image.
I hope this makes sense.
Kind regards
Lesley