Instagram has said more than once that saves and shares matter more than likes when it decides what to recommend. A save means "I want to see this again", and that's exactly the kind of content the algorithm wants to push.
Saves are cheap, quick and invisible to visitors (nobody can see who saved your post), which makes them one of the most natural-looking boosts you can add.
Best for: recipe posts, carousels with tips, educational reels, before/after posts — anything people would genuinely save. Combine with shares for a stronger push.