On the surface, Tags and Favorites can look like the same feature. Both allow you to 'flag' specific slides to reference later — but which one you choose will depend on what you want to do with the slides.
Favorites are like one big bucket full of bookmarks. You can't sort or group them, so they're most useful if you want to jump to a specific part of an existing slideshow.
Say you have a slideshow about pumpkins and the last four slides are solely about pumpkin soup. If your prospect is really into pumpkin soup, you can Favorite the first of those four pumpkin soup slides, and jump straight to the pumpkin soup section. Since the other pumpkin soup slides are already in the order you want, you wouldn't need to tag them separately.
Similarly, if you have a submenu all about sourdough, with links to recipes on separate slides or documents, you can favorite that submenu; then, you'd be able to jump straight to that slide and navigate to the relevant recipes from there. Again, you wouldn't need to tag that slide, because the navigation structure from that point already works for you and your prospect.
Favorites exist independently as quick reference bookmarks for certain parts of your presentations. But if you want to 'swipe through' your Favorites, or share them to a prospect as a set, that's when Tags come in handy.
To get a full rundown on Favorites, check out this Knowledge Base article.
Tags allow Viewer users to collate existing slides into a custom slideshow. They're sort of like mini-presentations, and allow you to create presentations that are completely tailored to your prospect and their needs.
If you find there are certain slides you never touch in a presentation, you can simply create a slideshow without them.
This will help you look professional and position yourself as the expert you are when you know the presentation back to front.
To get a full rundown on Tags, check out this Knowledge Base article.