Good question: why would anyone want to suspend someone in an erotic setting? Well, actually, there is a variety of reasons and answers to that question. What you may want to do is use this article to find out what it is that turns your crank.
Most often, when asked this question, people will answer: “Because it is spectacular”. And yes, that is true. It is spectacular. But the question arises: do you think “spectacular” is what belongs in your love life? Do you have fireworks and drum-rolls when you orgasm? In your head sure (hopefully anyway). But in your head is not the same as having a marching band in your bedroom. “Spectacular” is good, but it should not be a reason, at least not the only one. If it is, what you are actually doing is showing off – you are not having an intimate adventure with your partner
Suspension techniques are an integral part of Shibari (rope bondage). And as a result, they are best described in this Japanese context – including the reasons for suspending someone.
The most common reason is to change reality. Taking someone’s feet off the floor – even without changing their position towards gravity – is a powerful and effective way to alter the way in 
which you experience your surroundings. There is some uncertainty involved, simply because you are used to having your feet on the ground, and your perspective is (quite literally) different. In other words, the world around you looks different simply because the angle of sight changes. Changing the position in relation to gravity dramatically enhances this experience.
Another factor, to take into account, is uncertainty. People usually do not feel completely safe when their feet are off the floor. Since we are not birds, we are not used to being up in the air and this uncertainty brings another factor into play: the need to surrender and trust. When suspended, the person in the suspension usually has few other options but to trust the one, performing the suspension, and to follow his or her lead. This creates a powerful experience of dependency, something that most people with a submissive inclination will love. To the giver it brings about an intense feeling of power.
Concentrating on Shibari specifically, two other factors are involved.
First there is the use of Shiatsu erotic pressure points. Shiatsu in therapeutic terms will often mean the therapist will quite literally place (part of) his of her weight on the client since in some areas “the harder you push, the better your impact” is certainly true. Experience has learned that this is largely true for those erotic points and areas, that CAN be pushed on, for example the inside of the thighs and the many pressure points, along the spine. You simply do not have that many hands, so the ropes and the weight of the suspended person will come to your aid and provide intensified pressure in various places at the same time.
Secondly, never forget that in a traditional Japanese setting at least some level of cruelty will always plays a part. In this sense, the demanding and sometimes painful aspects of a suspended bondage play an important part too.
In other words, there is more than one level to Japanese suspension. These different levels are used in a carefully designed and planned mix.
Both these articles on suspension are great, well written informative and even funny. Something you don’t usually see in this genre.