Buying a toaster oven while already owning an oven is not necessarily redundant. It can be useful for a small portion, an additional cooking space or a task you repeat often. The question is whether those benefits occur frequently enough to justify another appliance on the counter.
List the meals you would make in it during an ordinary week. If you cannot name more than a vague possibility, wait until your routine becomes clearer. If you repeatedly heat one small item while the main oven is occupied, the separate cavity may solve a concrete problem.
Compare the complete fit
Measure the interior and the door opening, then check the cookware you plan to use. A baking dish may fit physically while being prohibited by its own manufacturer for countertop appliances. Confirm both the dish and appliance instructions rather than assuming conventional-oven compatibility carries over.
Rack positions affect the space around food and heating elements. A large number of positions is useful only if they suit your tasks. Read the recommended placement for baking, toasting or other permitted modes, and leave required clearance above and around the food.
Include the open door and ventilation space in your counter measurement. The appliance may need a location different from where you intend to store it. Keep cords, packaging and other objects away according to the manual.
Do not compare only the heating time
A smaller cavity may suit a small meal, but avoid assuming a guaranteed energy or time saving without measurements for the actual use. The whole routine includes setup, preheating when needed, cooking and cleaning. A tray that is difficult to wash can cancel out some of the convenience.
Check the crumb tray and interior cleaning access. Foil, parchment and liners have model-specific restrictions; covering parts of the cavity can interfere with operation. Use only arrangements permitted by the manufacturer.
If the appliance gives you a useful second cooking space, choose the size around that role. It does not have to replace every function of the main oven. If it mostly duplicates work you already do comfortably, the clear counter space may be more valuable.