See, that's where you're drawing a completely speculative conclusion though. The Humans wouldn't have to conquer the watchtower to carry on nefarious deeds if they were to do it the same was as the Orcs making tunnels without being seen by the Elves. Think about it this way - if the Humans loyal to Sauron had the guidance of Adar to work under the watch of the Elves, in many ways like Theo already does in finding that evil sword, then what would be the difference to what the Orcs are already capable of? They don't even need to capture the Elves considering they've practically waited them out long enough for them to completely leave. Had the intentional burning of villages been held off for say a month's time, the Elves would have been long gone from the area.
See how little sense there is in the plot overall? Everything is conveniently timed to support Arondir's story. Why have the Elves begin to leave the area? To build drama between Arondir and the Human female. Why are the orcs burning stuff now? Because the show needs a nefarious plot for Arondir to uncover. This is sadly how the narrative is being moved forward. It's all built around Arondir. The rest of the plot is very contrived when you consider how patient Adar has been shown to be with his tunnelling plans under the noses of the Elves. He could literally have kept them from burning villages until the Elves were gone and continued digging paths to every village before they even remotely attacked. But the show isn't really thinking about it from his perspective, it's all being built around Arondir instead, which is what leaves the motivations of other characters completely up to speculation and guessing, and prone to inconsistent or irrational decisions by them.
Which makes the whole narrative purpose of the watchtower contrived. You're explaining how purposeless it is in the story overall for me. I don't even need to tell you how pointless it is to be the center of this narrative, you already realize it.As for the tower's purpose - it seems that there was none, and wasn't it the reason why the elves were finally called back? It was a relic of old times, when the threat was real and the elves were afraid of Saruman's return. But obviously it wasn't happening, and the tower & the elves were there... because they were always there. This isn't THAT surprising, really; the tower had a purpose long time ago, that purpose was slowly diminishing. There always has to come a time when you deem something useless. The elves did that, calling the people back. We don't know the whole story & the past (and the show is slow enough without additional explanations of the history). One can assume that in the past, immediatelly after the war and Saruman's disappearance, the tower was only a single element of a bigger system, and with time, it was the only thing left. And the elves finally decide even that last thing is worthless.
Why is the watchtower important in Rings of Power? Because they needed a reason for this Arondir Elf character to exist in the Southlands and a setting that makes sense for him to be established there. That is it.
The watchtower and the Elves stationed there are otherwise set up to fail. They were never really there to guard the place narratively, they are merely there to support Arondir's story and be fall-guys that make his story feel all the more tragic and dramatic. The way it's actually played out in this story , they're nothing more than security guards whose duty is to observe-and-report, and they aren't even able to observe anything useful until it's already too late.