Actually, I think I misunderstood what you meant by "circle movements." Perhaps what you meant to say was that Hapkido, like all internal styles, has a greater focus on the use of small and subtle circular motions than external styles like Taekwondo.

Internal styles will use small and subtle circular movements to manipulate rotational force (torque), as well as the use of other biomechanic principles like leverage etc. This video demonstrates how you can counter your opponent's power just through the use of small and subtle rotations.

Having said that, not all internal styles use small circular movements though. Most do, but some don't. An exception that comes to mind is none other than Hapkido's forerunner, Aikido, which uses quite outwardly large circular movements (even their wrist and ankle rotations are rather "large"), as can be seen in this video. However Aikido practitioners justify this by saying that Aikido techniques were based on Taijutsu used by Samurai, this should be able to be used when holding a sword, thus you're unable to do the smaller and tighter turns that other internal styles use, lest you run into your own blade! ;D

But I personally prefer smaller, tighter and more subtle circular movements.

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Note: I'm not saying that circular movements or styles with more of an emphasis/focus on circular movements are inherently superior to linear movements and styles that emphasis those. While circular movements have the advantage of being able to manipulate your opponent's power, linear movements have the advantage of being faster (since the shortest distance between any two points is a straight line!). In reality, all martial arts have both circular and linear elements to them and both have their values in practical application. While I enjoy discussing different elements between styles, I'd like to remind people that by doing so I'm not encouraging or condoning "style discrimination." As I've often said before, the best style is the style that works for you (the individual). Besides, my favourite move would have to be a straight external and linear kick to the janglies!