Work that needs strength or cleanliness: camp chores, batoning and splitting kindling, field dressing game, food prep outdoors, and any job where a pivot would clog or a lock could fail. There are no moving parts, so there is nothing to break and nothing to clean out.
A fixed blade is one continuous piece of steel from tip to handle. A full tang version has no point where the cross-section drops, which is why it survives lateral load and impact that would damage a folder.
That makes it the right tool for batoning, light prying and anything involving force.
Field dressing, filleting and messy food prep fill a folder's pivot with material that is unpleasant and difficult to clean out. A fixed blade washes under a tap.
This is why hunting and fishing knives are overwhelmingly fixed blades.
Carry. A fixed blade needs a sheath, takes up more room, and is more visible. Carry rules also vary by state and locality, so check yours before carrying one outside your own property.