What are benefits of the pre-order playable characters, Catwoman and Robin?

First off, it's worth mentioning that while Robin is indeed a pre-order bonus character, as detailed in this site (he's a pre-order bonus in BestBuy in the US and GAME in the UK), Catwoman is not a pre-order bonus. Catwoman is only available for people who purchase the game new or pay extra for a key once they buy a used copy. See this article.

Neither effect the main plot directly. Robin is a challenge only character:

exclusive Robin character with his own unique gadgets and moves. Playable in all challenge maps of the game, including two additional maps that are only available with Best Buy preorders (Black Mask Hideout and Freight Train Escape).

And as for Catwoman, it is best to have a look over here:

Playing as Catwoman is not required to complete the game.

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Catwoman is a unique character with her own story, moves, and weapons.

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Without the VIP pass code, Catwoman still appears in the game as part of the story, but her playable missions remain locked.

EDIT: While she does not appear to affect main story, technically, she does change the way the game begins and the amount of Riddler challenges (from here):

Get that Catwoman DLC, and get it before you start... because it will affect how the game begins. The new Catwoman intro isn't great, but her later missions are good (plus, she's fun to play as, as you can see here).

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There are 400 Riddler things to do even if you don't have the Catwoman content (at least 40 more if you do).

Catwoman kinda has an effect on the storyline, Robin does not.

Catwoman has her own sideplot (consisting of 4 "sections" or missions) which expands/adds to the story in the game, but it doesn't (afaik) have any noticeable effect on the main storyline of the game.

Robin on the other hand doesn't have any relevance to the main storyline and is just there as a playable character for challenge maps.