Why is this a successful business strategy?
Because people are weak that's why. They have a bad case of FOMO (fear of missing out).
If they load up a game and see there is missing content, it will mess them up. They will keep wondering about the new content and feel that they are missing out. They might even end up buying it but never using said content yet they would have the peace of mind that they can experience the content if they need to.
This is actually worse for competitive games. As a competitive player, you can't justify NOT buying new characters the day they come out. A new character could be busted and you could have an edge if you lab a new character. How many times have NRS put out a busted DLC character? Even if a character isn't busted you would have to download the character to check to see if they are busted or learn the match up.
And companies feed off of this. It is actually a fuck ton better for them if you just buy characters. Think about it... Marvel Infinite costs $60 and it's coming with 30 character so that's $2 per character. They are going to be selling new characters for $5 meaning they made better money off of individual characters. Imagine they put out 20 characters over a span of a few years... that's $100 you have to spend on the game yet you are getting less content than what you paid for the full game.
Even if not enough people buy them, they still come out on top.
Edit: Oh you were taking about showing off content at the correct time. Enzo has the correct answer on that. Showing off DLC way before a game's release is looked down upon heavily by gamers.