What's the difference between a convection countertop oven and a sous vide cooker?

They solve different problems. A convection countertop oven uses hot circulating air to bake, broil, roast, and air fry. A sous vide immersion circulator heats and circulates a water bath to cook vacuum-sealed food at a precise temperature for tender, edge-to-edge doneness. Some shoppers buy both; a few high-end combi ovens attempt to do both jobs, but the consensus across specialist communities is that dedicated tools still outperform combo units.