Until Citizens United blew the door open, if you were a corporation you had to find ways to get money to candidates.
Telling your employees to donate to $CAUSE was one of them, and offering them bonuses or matching or other "incentives" -- aka "sign here to get your X-Mas bonus" on donation form -- was a way to move money.
Indeed, it isn't your employer's business what or whom you donate to. But the more straightforward answer is that they are likely required to ask by law (possibly subject to other criteria).
That's a good question, because I've had no luck finding an example.
I do recall reading about it, but that could have been decades ago. And it's hard to find old stuff.
What I find is about contributions from employees of government contractors, and from government employees. So maybe that's been the driver for the requirement.