DNA standard is like a control because it helps to identify the weight of the unknown samples run on electrophoresis gel. It helps to compare the bands of the unknown sample with its own band positions, and we can get the relative weight. DNA ladder consist of many bands whose sizes are known already, so that they can help to find the size of the sample DNA.
Also, a DNA ladder can't be altered during the process of running the DNA in electrophoresis gel.