The addition of a spreader bar gives a hammock a wider and flatter lying surface. This makes it far easier to get into and out of, which is perfect for the hammock novice.
They are designed so that the hammock user can lie lengthwise, rather than the diagonal position of the non-spreader bar hammock. This gives a shadow free experience and is perfect for sunbathing in and for short naps, but perhaps not so comfortable for a long sleep.
If they are not hung tautly there can be a danger of them spinning or flipping over. The tighter the hang the more comfortable they are too. They require a bit more space than a non-spreader bar hammock to give the perfect hang. The hanging points should be at around a quarter of the total length. EG: if you have a hammock with a total length of 4 metres, then it should be hung at 1 metre off the ground.