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).
The above is relation to presidential campaigns specifically but the logic applies equally to other electoral contests.