Why pay a fee to have your article (in a subscription-based journal) made open access when you can just put the preprint on arXiv?
If you work in a field in which people use and trust the arXiv, then there's little reason to pay fees for hybrid open access.  One reason would be if your funding agency requires open access publication and does not consider the arXiv to be an acceptable substitute.  Another would be if you wish to encourage the journal to transition to fee-based open access publishing.  In your case, I doubt either of these reasons would be compelling.

From the publisher's perspective, it's a no brainer: why not offer authors this option?  It can't do any harm, since it pays for itself.  If few authors choose to pay the fee, then the publisher can use this as evidence that there is little demand to change their business model.  If many authors do, then it smoothens the transition to becoming a fully open-access journal.