Quick take
Canva is the tool I would give a non-designer who needs useful visual assets today. It is good for social posts, thumbnails, simple decks, flyers, ads, lead magnets, and quick video clips. The trap is thinking the template has done the thinking for you. Canva can make something look finished before the message is actually clear.
Best fit
Use Canva when the job is visual, repeatable, and tied to a channel. It works best when you know the audience, the format, the brand rules, and the one action the asset should drive.