This upgrade is very useful in Deflation Mode, Daily Challenges that have a tower limit, and in Clock, Switch, The Eye, Country Road, Banana Factory and Floating Islands, because the player can keep changing their towers without losing any money if are not satisfied.
A second "strategy" associated with Monkey Tycoon is to sell your towers and then re-purchase them within the radius of a Monkey Village. The necessity of Monkey Tycoon is to ensure that you do not lose money when selling.
A similar strategy requires towers which have abilities to be sold and re-purchased for the ability to be used straight away again.
You can also sell the towers you don't need (example: MOAB Maulers and MOAB-SHREDR Spikes at round 63) and buy banana farms with that money, then sell the banana farms and buy these towers again.
Allows Abilities to be used over and over again, like Ground Zero. However, it will not make Supply Drop be used over and over again, because it has a cooldown after a Sniper Monkey is upgraded to Supply Drop.
In Bloons TD5 Mobile, this can only be "obtained" via the Monkey Lab, by upgrading Sell Price, even though at max level the percent of refund is not 100%.