Big deal, it was the first game to have a place where you go and battle that way. W/e if it's frontier or tower.
There is a difference between a Battle Tower and Battle Frontier. One being a Frontier is more of a group of battle facilities with different Frontier Brains leading them. Each facility has its own rules of battling making it different everytime for the player, while the Battle Tower has kept its same battling style since Crystal.
So, there is a deal between the two as well as difference. Releasing the news on it returning however was to show the improvements of the game since the original Crystal. Like said before it was also a way to attract the players, more so those who fancy the Battle Frontier so they'll know they have a new challenge waiting ahead. It extended the idea of the Battle Tower for players to compete against, and because of its popularity, having it known to make an entrance for the Johto would give fans the hype to purchase the game even more.