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It doesn't, really. The only difference is the venue where it's prosecuted.

It at least affects maximum sentence, right?

Fraud terms vary between states, but you're already looking at 10-15 years for commercial fraud before aggravating factors, from what I can tell (I'm not a lawyer).

It does influence whether there's federal jurisdiction.

The idea behind treating mail fraud separately is explicitly that the severity, if not the legality, of fraud should depend on whether or not the mail was involved.

Remember that when mail fraud came into existence as a discrete crime the mail was the primary means of communication for essentially everyone and everything. Tampering with it went beyond deceiving any one person or group of people.