In statistics, variance is a measure of the spread of a set of data with respect to the average value, or mean. Mathematically speaking, variance is the sum of the squared difference between each data point and the mean  all divided by the number of data points. More simply, variance means getting some results or data points that deviate from the average or expected result and representing that difference numerically. This can be an advantage, a disadvantage or both.
Statistical Surveys

Finding variance in a survey data set is typically considered a good thing. It can be a sign that a survey was set up to correctly take a range of information from respondents. For example, a survey of yesorno questions may not provide much detail about the subject of the questionnaire. However, a survey on the same subject with respondents choosing from a range of answers offers more information  and a greater chance of variance. Variance could be seen as a disadvantage only if the surveyor saw results that deviated from a desired outcome.
Business

In business, variance is often referred to in terms of accounting with respect to costs. For example, the actual cost of doing business might vary from the estimated cost. Obviously, this can be an advantage if actual costs are lower than expected, and vice versa if the opposite is true. Whether an advantage or disadvantage, cost variances in business should always be assessed and the cause or causes or the variance should be determined.
Clinical Trials

During clinical trials of drugs or medications, the scientists involved often have a desired outcome, and variance from this outcome is typically seen as a disadvantage. There are three main possible causes for variance in this context: factors related to the preparation and collection of the sample, improper calibration or precision, and inherent biological variation  such as the test subject being in a natural variation of the life cycle like puberty or menopause, according to Callum Fraser, a biological variation expert from the University of Dundee in Scotland, in his Westgard QC article "Biologic Variation: Principles and Practice." Trial administrators need to be mindful of these potential disruptions to their work.
Genetics, Phenotype and Evolution

Being the foundation of evolution, genetic and phenotypic variation is typically seen as advantageous to life on Earth. The source of phenotypic variation is typically an acquired trait that has an evolutionary advantage, such as an animal's ability to adapt to the loss of its natural habitat. This kind of genetic and phenotypic variance can be a disadvantage when it comes to antibiotic resistance, as certain deviant strains of bacteria have emerged that are resistant to penicillin and other lifesaving drugs.
References
 QuickMBA: Standard Deviation and Variance
 Khan Academy: Variance of a Population
 SurveyMonkey: A Survey Resolution: Stop Throwing Away Your Variance!
 Inc.:Variance
 Westgard QC: Biologic Variation: Principles and Practice
 Nature: Why are the variable levels and patterns of genetic variation important?
 Photo Credit shironosov/iStock/Getty Images