No, it is not necessary to preheat a slow cooker before putting ingredients in. Slow cookers are designed to cook food at low temperatures over an extended period, typically several hours. They work by gradually heating up the ingredients and allowing them to cook slowly.
In fact, preheating a slow cooker could be potentially unsafe. Unlike ovens, slow cookers are not designed to reach high temperatures quickly. Starting with a hot slow cooker could cause uneven cooking and may even lead to food safety issues.
To use a slow cooker effectively, simply place all the ingredients into the cooker when it's cold, and then turn it on to the desired temperature setting and cooking time. The slow cooker will gradually heat up and cook the food evenly over time, resulting in tender and flavorful dishes.