Increasing PPM would have to lead to a higher passive credit earnings, as long as shops have stock to sale. The more people randomly delivered throughout the Tiny Death Star, the more people who buy items from shops.
How can we determine that PPM applies to passive credit earnings and not player activity? As there is no way that the level 2 elevator, the NB200 Commercial Lift, the first available upgrade for the elevator in Tiny Death Star, is generating 8 people per minute for the player to deliver. This rate means that the player would be present with a stream of bitizens spaced every 7.5 seconds after the delivery of the last bitizen.
Examining the second elevator upgrade, the Insta-Lift 2000 SE, we can see it has a 12 PPM rating. This means a bitizen would have to be popping up every 5 seconds in the lift to be escorted to a level by the player. So we can conclude that if the information posted in Tiny Death Star has any in-game meaning, then it must apply to the background bitizens that are arriving on the floors.
For further information, we can look at the Tiny Tower Wiki about elevators here: http://tinytowerwiki.com/Elevators
It notes that PPM = Passengers Per Minute, the rate that people are delivered. Since this rate is much too quick for passengers to be delivered by the player, it must apply to passive delivery (bitizens being delivered by the elevator "in the background").