I think it's the lower entry next to ticketing or baggage claim.
People with checked baggage naturally go down to baggage claim, which has large open areas and a view of the escalators, as someone else pointed out. However, people without checked luggage may not think of going down to baggage claim, so it may be easier and more convenient to just meet them upstairs in the big open area in front of the TSA security check. However, if you meant curbside, the same checked-luggage consideration can help; arrivals (downstairs) with checked luggage and departures (upstairs) without checked luggage.
Lower entry to the left of ticketing, there is an escalator that comes down into a large area. Very easy to spot people coming down from their flights.
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