Different sampling schemes
Two different ways to collect a random sample of n values from a finite population of size N are common. In both sampling schemes, each population value has the same chance of being in the sample.
Since a SWR may contain the same population value more than once, it covers less of the population than SWOR. SWOR therefore gives more accurate estimates of population characteristics.
Large populations and/or small samples
If the sample size, n, is much smaller than the population size, N, there is little practical difference between SWR and SWOR — there would be little chance of the same individual being picked twice in SWR.