Understood that that's the conceit, but referring to purely the initial season on Netflix, it would be perfectly substitutable to have Evans speaking not /through/ a Sophon, but /to/ a Sophon -- a sufficiently-intelligent, sufficiently-aligned computer that can represent the San Ti interests fully, and react as a San Ti would. Nothing that has happened in the story /so far/ requires a real-time update on the status of the San Ti fleet, or otherwise an actual round-trip communication; it could all be incorporated into the San Ti model of a sufficiently advanced AI for the purpose of representation. (Even as an AI skeptic, I do think that such an AI is less of a technical reach than FTL communications.)
The San Ti need information from Earth as part of their plan to relocate there. At light speed, this would take 4.5 years to arrive. Without an FTL link to earth, they will be arriving "blind" (or with only the information that Ye and Evans might have passed along after 1967).
Fair enough. I suspect that this becomes obvious slightly later in the story. The way that the first season is presented on Netflix, there's really a bicameral distribution of possibilities here -- either the Sophons successfully suppress human tech development and the San Ti have no issue doing whatever they want even without realtime information due to technical edge, or they fail and no matter how much information the San Ti have it won't overcome the tech deficit. I'm sure that for story purposes exploring that negligibly small middle part of the distribution where tech levels are comparable at meeting time is most interesting, but as presented it doesn't feel like the San Ti's probability of success changes enough with FTL communication to suggest that they would take different actions in a universe where everything is the same except FTL comms are prohibited.
They are in-flight towards Earth when they discover the humans have the capacity to lie (0). This forces an immediate reappraisal of their plans, which can only happen with FTL communication.
Whether this changes their chance of success is certainly debatable, but it definitely changes their behavior.
(0) note that AFAIR from reading book 1 just about a month ago, this is series-canon, not book-canon.
When Evans speaks with the San Ti/Trisolarians, he is talking to them via one of a pair of sophons - quantum entangled protons.