A toaster oven is, without exception, much smaller than a conventional oven. This may not be an issue if you have a kitchen with a conventional oven built in.
A toaster oven can take up a lot of valuable counter space, but it can also be installed under cabinetry to minimize its surface impact. Conventional ovens are best for cooking large items like roasts and full-size pizzas, which most toaster ovens can't accommodate.
For very small living spaces without full kitchens or for break rooms, a toaster oven allows for smaller-scale cooking.
While old-style toaster ovens that basically just toast and broil are still around, many modern toaster ovens are also convection ovens. The convection setting combines heat with air circulation to create a cooking environment that cuts cooking time compared with a conventional oven.
For example, some convection toaster ovens are designed with the capacity to roast a small whole chicken; if it takes an hour and a half for the chicken to roast in a conventional oven, it will take just an hour in the convection toaster oven.
Although a convection toaster oven cooks faster, one of the big advantages of any kind of toaster oven over a conventional oven is that it gives off very little heat. Conventional ovens, on the other hand, are notorious for heating up the whole kitchen, making them almost unusable during hot summer months.
With a toaster oven, you can bake a pie in hot weather without significantly changing the temperature in your kitchen. Of course, if you bake a lot in the winter to keep the kitchen toasty warm, you won't get that with a toaster oven.
Cost and Efficiency
Toaster ovens, including convection toaster ovens, are much less expensive than conventional ovens. Conventional ovens are major appliances, and they can also serve as stove tops.
Toaster ovens are small appliances, with price tags starting as low as around $30 in 2011, up to a few hundred dollars. Conventional stoves start at around $400 and go up into the thousands of dollars.
Toaster ovens are also much more efficient, especially in the summer when a heated kitchen forces your air conditioner to work harder. In both the short and long run, toaster ovens have a much lower cost.