why do I need a dishwasher aerator

Are you  talking about a dishwasher faucet or a dishwasher machine? If it's the former, an aerator is a great water saving feature. It is such a cheap part that it would make no sense not to install one.
If you 're taking about a dishwasher appliance, then the device is not an aerator but an air gap. It prevents backflow of sewage into the dishwasher in the event that the sink to which is connects to (usually via the disposer) fills up higher than the level of the dishwasher's discharge hose. It is a very useful device in any case but it's obligatory to have in many parts of the country. FL and CA require it, and you can lookup your state's requirement in its Plumbing Code:
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The air gap isn't an expensive device either and it's not too difficult to install. Except of course if you need to make an extra hole in the counter top.  But, assuming that you've already had one, you should be able to reuse it with the new appliance.   
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