Why use year over year to compare metrics?
YoY tells you how much has changed between two points roughly 12 months apart. You can also compare data day over day, month over month, or quarter over quarter if those periods are meaningful for your purposes.
Sometimes, YoY can show you the direction of growth or shrinkage in a metric, or you can use it to demonstrate seasonality if you're comparing quarter over quarter (QoQ) or month over month (MoM) instead. It's a very simple comparison that says a lot and a way that you can string multiple years of data together to tell a story about your company's past and potential future trajectory.
Year over year versus year to date
Year over year is an important way to slice data but it's not the only way. Another common way people look at financial data is by using a year-to-date metric. These sound very much alike, but they couldn't be more different.
Year over year looks at a single data point in time -- say, the percentage of tenants who renewed their leases in the third quarter, compared to the prior year, or how much income the company made in the fourth quarter, compared to the year before.
Year to date, on the other hand, is cumulative data, and looks at the total metric to the point at which you're ending your analysis -- for example, how much income the company made from Jan. 1 through Oct. 14 or from Jan. 1 through April 21. Whatever your date, the metric is only cumulative to that point.