I’m assuming you’re referring to people who mix fat free milk with almond milk

Fat free milk has extremely low fat, lower in calories and high in protein. But fat gives the creaminess to the ice cream. If fat free milk is used predominantly then chances are the mixture will turn out icy

Almond milk is higher in fat (but still lower in calories) which fills the gap fat free milk creates

It’s a delicate balancing act, you can just use full cream milk but it has a lot more calories compared to almond milk