when do you use variance. is it a measure of spread?
Yes it measures spread!
You cannot add SD but you can add variances. Remember the Pythagorean theorem of statistics with the different SDs!
Variance, also referred to as s^2, is the squared value of the standard deviation.
Here’s a more formal definition: The variance is a numerical value used to indicate how widely individuals in a group vary. If individual observations vary greatly from the group mean, the variance is big; and vice versa. It might be used in explaining the spread of data, when explaining the linear regression of a set of data, or when using expected value in the probability section.