Why would an innocent man need a pardon in the first place? Tis a mystery....
Besides the already alluded to possibility of an innocent person being convicted, there's also the situation where someone is
legally guilty but
morally innocent (for example, someone convicted of murder whose "murder" was putting a loved one out of their misery at the loved one's request because they were too weak to do it themself). Someone in this situation could well ask for a pardon on the grounds that they simply don't deserve to be in prison.
Of course, neither of these apply to Trump.