The leading reason why people choose an inserts is there increase burning efficiency/output.
Using a blower equipped with the insert can really allow you to benefit. Look for unit with at least 80% efficiency for the best pay off.
Inserts can burn wood, gas, corn pellet, electricity
Other common reason to consider a insert if your fireplace has cracked flue tiles, holes in the smoke chamber , the damper doesn’t work etc…. the needed improvements to bring your fireplace to today’s standards could cost as much as or more then a new inert. Something to consider….