The understanding of the usefulness of the XIRR function in Excel has different reasons. There are some that would find it useful for one thing, while it might be useful for other things. It is practically useful for anything that has to do with calculating your returns.
When it comes to investing money, there are actually differences in time value. When depositing and withdrawing money and then receiving dividends, it becomes much more difficult to make the calculation of the annualized returns.
This is because there is certainly a difference between making an investment of $1,000 in January and investing the same amount in December, right before the year ends. This makes the XIRR feature of Microsoft Excel something that simplifies the calculation.