Compared to Smart Matches™, Instant Discoveries™ do not find any additional information. However, they let you apply information or photos to your family tree with just one click, which is a completely new functionality.
Doing the same process with Smart Matches™ would have required manual addition of people to your tree, one by one. So a single click to apply an Instant Discovery™ can save you hours of work you would have done manually using Smart Matches™.
Adding information manually is recommended for more experienced genealogists, since it maximizes accuracy, and lets you pick exactly which information you trust to add to your tree and which you'd rather keep out, but it's often too difficult to follow for amateurs and newcomers to genealogy.
So Instant Discoveries™ make it easier to add new information to the tree. Professional genealogists can still enjoy Instant Discoveries™, for example to add entire branches or profile photos to their tree from sources they fully trust, while using Smart Matches™ for other needs that require closer scrutiny.