The HRA is a reimbursement arrangement that is entirely funded by your employer. It is essentially free money to you, reimbursing you for medical expenses not covered by your insurance. However, there are a few situations where it might make sense to opt out of this benefit.
If you have an HSA-eligible High Deductible Health Plan (HDHP), you can make tax deductible contributions to your own HSA. However, if you are covered by an HRA, you are ineligible to contribute to an HSA. Therefore, if you want to contribute to an HSA, you would need to opt out of the HRA.
Similarly, your own HRA automatically covers your spouse. So if your spouse wants to contribute to an HSA (or even if your spouse's employer is contributing to her HSA), you would need to opt out of your HRA.
To decide whether an HSA is more advantageous than your employer's HRA, you need to look at how much of a tax deduction you would be getting by contributing to an HSA and compare that to how much your employer is offering in the HRA. Keep in mind, however, a big difference between the two plans: An HRA belongs to your employer and is entirely funded by them. Anything unused goes back to them when you leave. With an HSA, however, the money inside is yours to keep forever until it is spent by you.
One other note: There is something called a limited-purpose HRA, which is an HRA that can only be used for specific types of health expenses, including dental and vision, but cannot be used for general medical expenses. This type of HRA does not make you ineligible for the HSA.