what's so great about Marcus' gift? like, what makes it so beneficial to Aro? if Aro can already read every thought a person has had, why would he ever need Marcus to tell him (checks notes) ah yes, "these two are dating"? I can't work out what's so powerful about Marcus and I just wondered if you'd break it down for me
Ah, but here's the question, what are thoughts and what is the truth?
Aro reads every thought a person has ever had, he knows you better than you know yourself, certainly better than anyone else ever will. But he sees the world the way that you see it. What if you're wrong? What if you're biased?
Marcus, presumably, sees relationships as they are. He sees the network of relationships to a person: all the people connected to you in ways that you might not have ever recognized or acknowledged even to yourself. If someone is in love with you, and you're clueless, Marcus sees that in an instant; Aro may not pick up on it.
Marcus also sees all of this very quickly and at a distance. In early days, when the Volturi were actively at war with the Egyptians and the Romanians, this provided vital intelligence when Aro couldn't physically touch someone.
Marcus was probably the one to inform Aro that Amun was the weak link in the Egyptian coven: he would sell out his coven in return for his own survival.
Still, even in the modern age when you can't say "no" to touching Aro's hand, Marcus' gift is still very useful for the first reason. It provides very fast intelligence from a different angle than Aro's. Marcus can support or refute conclusions Aro draws from his own gift.
Case in point, Edward Cullen.
We don't know exactly what Aro sees in Edward's head, but presumably it's 100 years of Midnight Sun. Aro likely walks out of that brain thinking that something's not right, and he can guess what really happened, but it's hard to tell what's true and what's not. Especially where Bella Swan is concerned. Does this girl truly love Edward, as Edward believes? Is Bella Swan even a person at all or just this empty shell of a being Edward has projected upon? And what about the coven in general? What are their actual relationships? Surely, it's not whatever Edward thinks it is.
Aro actively touches Marcus' hand in meeting Bella in order to gain that vital second opinion. What the hell is up with these two? And the answer, well, whatever he saw made him say, "Wow, can't believe you haven't eaten her yet, Edward. Wow." And Bella thinks, "Ah, yes, our love is amazing."
TL;DR Aro sees us as we see the world and ourselves, Marcus has the ability to see what we ourselves do not see or do not acknowledge