The 'tower' is shown in posters and the series as an observation point to provide early warning of an attack. But OK, when did we see that happen in this series where the tower was used as a base of defense? We didn't. The elves got captured, meaning whatever it was designed to do, ultimately it failed. The fact that it happened off screen means we can only guess how it happened and whether or not the tower actually served that purpose or not as you describe it.
It is an alternative to Eregion becasue the core of the narrative of the creation of the rings of power was centered on Eregion. That is where most of the key events took place and Eregion was created explicitly for the purpose of being a bulwark against Sauron. This is why he targeted it and used it against them. Therefore, if they have made up this new tower as a key part of the narrative for this part of the second age as a new bulwark against Sauron then it is an alternative to simply focusing on the story of Eregion as already told in the lore. But this series is more interested in keeping things mysterious than actually telling a story. So the tower didn't serve its purpose in the world as it should because the writers needed to keep the presence of the orcs a secret, while at the same time showing the audience that the orcs are very real and a threat. But in showing the audience that the orcs are real and a threat, they are also telling the world of middle earth as well, which is why they had to come up with these contrivances to keep their presence still somehow secret.
Also, keep in mind that Sauron was able to appear at Eregion because the Haradrim and other forces he was amassing in his armies were kept far to the South and out of view of Eregion, the Elves and the Numenoreans. It was a key reason why he was able to arrive at Eregion in disguise and not be suspected(except by Galadriel). Once he is revealed as Suaron, he musters his forces to attack and there is no more secrecy. So the secrecy in the original lore served a logical purpose and moved the story along. The secrecy here in this series really serves noting and doesn't really move anything along because their activities don't signal Sauron's return to the wider world. And it cannot because it would be hard to have Sauron show up in disguise while having his presence revealed and known to all in middle earth. They would be wary and on their defenses. But we will see how this series plays out.