Nothing . Acumatica will allow you to proceed with processing even if the address isn’t validated. 

I was aware of that, but what downside - if any - are there? If there are no downsides, then why bother? Why even have validating addresses as an option then?

It could save you money if you have a lot of carrier fees for address corrections. For example, UPS might charge you $20 for each shipment they need to correct the address. With LTL carriers it might be even higher. 

It would depend on your business situation. Personally I don’t see it a lot, maybe $200 in fees throughout the year. 

Thank you.