After you ligate your fragments together, you transform the DNA into bacteria, plate the bacteria onto growth media, and wait for colonies to grow. In the transformation step, not all of the bacteria in the sample take up DNA. In fact, only a small percentage take up DNA. Therefore, when you plate cells onto a agar without any antibiotics, you will get a lawn of bacteria, most of which have no plasmid at all. This is the main reason why you need antibiotic selection: to be able to distinguish bacteria with a plasmid from bacteria without a plasmid.