Backing rod  is guaranteed sufficient dilatation joints and proper function of sealant. It is important, that the sealant is assigned only in 2 points.The middle space in joint which remained free is for the possibility of movement and expansion. This will allow flexible PE sealing cord, to which the sealant will not stick and can thus move freely around. Another advantage of using backing rod is also a saving of material, which is compared with the cost of sealant more expensive.
This does not mean that we can only replace the sealant with backing rod. It should be observed as a thick sealant deposited on a cord. Always depends on the overall width and depth of the joint. Among the common errors are applied very thin layer of material on the cord, which after drying causes the so-called contour lines circumscribe and later cracking grout above backing rod.Backing rod is not used in places where the gap is less than 1 cm. There shall  be used only sealant foe example around windows, doors, into small cracks, etc. ..