An insulated bag does not necessarily need ice to be effective. The primary purpose of an insulated bag is to provide thermal insulation, which helps maintain the temperature of its contents. This insulation works by reducing the transfer of heat between the interior and exterior of the bag.
If you're using an insulated bag to keep items cool, adding ice can certainly enhance its cooling capabilities. The ice acts as a cold source, helping to lower the temperature inside the bag and keeping perishable items chilled for a longer period. Ice can also help absorb heat that enters the bag over time, further helping to maintain a lower temperature.
However, an insulated bag can still provide insulation and help retain the existing temperature of its contents without ice. For example, if you have pre-chilled items or use cool packs or frozen gel packs, the insulation alone can slow down the rate of heat transfer and keep the contents cooler for longer.
Ultimately, whether or not you need to use ice in an insulated bag depends on your specific needs and the desired temperature maintenance. Ice can be beneficial for enhancing cooling, but the insulation itself is what primarily helps preserve the temperature.