Most bodybuilding type movements are functional, because otherwise we would not have a certain muscle. Muscles are used, or else they do not exist (or, if not worked, shrink). So yes, a bicep curl is as functional a movement as lateral delt raises (this is how you push your car door open from inside, and also how you nudge away defenders in soccer or other sports, for instance). However, biceps are a small muscle group, and typically work in unison with other stabilizing and/or larger muscle groups. Because it is so small, it does not need to be worked to the same extent as a larger muscle group, like your back or legs.
Personally, I work my biceps (and triceps and shoulders) directly for only 3-4 sets, 1x/week. However, each of these groups get worked indirectly from other exercises I do, as I work my larger muscles (chest, back, legs) 2-3x per week. incline and flat bench presses through the full range of motion will work your anterior deltoid and triceps, chin ups and pull ups will work your biceps and posterior deltoids, and rows will work your biceps and posterior deltoids as well.
So, in short, most every bodybuilding exercise is "functional in the practical sense of the word" because every muscle in our body has some function (otherwise it would not exist). However, a bicep curl is not "technically functional" because it doesn't work other muscles in unison (aside from your core) as it is a single joint movement. Typically, "functional" exercises are ones that operate over more than 1 joint, such as a squat, which flexes through your hips and knees (and ankle) joints.
I personally consider calves, biceps, and triceps (and to some extent, deltoid) work "supplementary/accessory" exercises to be treated as such.