When purchasing SFV AE, you receive the codes for the season pass for S1 & S2, thus giving you the characters & their premium costumes.
This is ultimately done as a step for authentication purposes - the current vanilla version of the game will be indistinguishable to AE in regards to them being a client from the server's perspective, meaning the game wouldn't know whether or not you are someone who purchased AE or someone who had their base game upgraded via a patch. And since Capcom isn't giving S!/S2 away to base purchasers without them purchasing that content, either as passes or through FM or individually, then the servers need some other way to authenticate you were an AE purchaser and not a base edition user. Hence the vouchers.
Most likely, the actual physical disc will have the client version needed for the Jan 16th launch, meaning you won't need to download all the S1/S2 characters & patch the game the way the base users have done.
Now I know someone is going to just ask "Why don't they just verify the date your CFN account was created instead of giving vouchers in order to authenticate you're an AE user?" - simple answer to this is that creating CFN accounts costs you nothing. I can make a myriad of PSN accounts on my PS4 & run the digital copy on my machine, and create a CFN profile on each one. Or if what they are looking for is CFN profiles made after Jan 16th, I could merely buy the vanilla game today on a super crazy sale for $!5, not make a CFN profile, then make one on Jan 16th after the game has been updated & get the S1/S2 characters.
The only reasonable way to give out that content without some form of authentication as to whether or not a user is a base SFV user or SFV AE user is by making all of the S1/S2 content free for everyone, which would be both a major burn to the users who purchased those season passes at full price, and it would severely destroy the value proposition of the S3 pass.