Curls and other exercises that primarily work the biceps (brachii) have something of a bad reputation; they’re frequently regarded as being non-functional exercises for insecure gym bros whose only purpose is to look good in the mirror when flexing. I’m not sure why this is the case, though, because bicep curls (with a curl bar or with dumb bells) are actually quite functional.
So, when in normal life does one pick up something a bit below one’s hips and bring it up to one’s shoulder? One does exactly that thing when picking up a child who is old enough to walk. Admittedly, sometimes one has to bend over a little because their armpits are closer to knee height than to waist height, but it becomes a bicep curl once you stand up.
The most common way to pick up a child who can walk is when they stand in front of you and lift their arms up to indicate that they want to be picked up, in which case you tend to use two hands, one under each armpit. Sometimes you’re already holding something, though, and so you need to pick then up with only one arm. This is when the bicep curls really come in handy, since all of the child’s weight is being lifted like a dumb bell. (Pro tip: have the child lift it’s leg so you’re picking them up by their femur while they hold onto your upper arm with both of their arms. If you try to do it under just one armpit it will probably hurt them unless they’re very little or can pull down with that arm hard enough that their latissimus dorsii flexes hard enough to bear the weight.)
Fun fact: little children enjoy when you do reps of bicep curls with them, though in my experience they tend to max out at around 5 reps before they want you to just hold them like normal.