I went up to Jerusalem with Barnabas, taking with me also Titus… But not even Titus, who was with me, being Greek, was compelled to be circumcised. - Galatians 2:1,3
To support his bold claim that circumcision is no longer necessary for gentile Christians, Paul further claims that the saints in Jerusalem had met Titus without requiring him to be circumcised. For the second claim to support the larger claim effectively, Paul must have expected the Galatians (1) to know who Titus was, and (2) to believe that Paul's claims about Titus were true. The simplest explanation for Paul's apparent expectation is that Paul sent Titus to carry this letter.
Supposing Titus delivered Galatians can also explain why Paul references Titus without identifying distinctions. That is, neither "who was with me" nor "being Greek" denotes a unique referent. These phrases do not qualify as identifying remarks because many people could have been with Paul and many people were Greek. I return to the possible meaning of these two phrases below.
For context, please observe that Paul has obvious reasons for failing to identify Peter (1:18) and John (2:9), who require no introduction as Jesus's apostles, and also Barnabas (2:1), who requires no introduction because he helped Paul found the Galatian assemblies. As for James, "the Lord's brother" (1:18), Paul's identifying descriptor was undoubtedly mere disambiguation, supposing at least one of the following: e.g., that the Galatians had heard about James the apostle in the same stories they heard about Peter and John, or that James's prominence in Jerusalem was widely known, or that we take Paul to imply that Galatia's other recent visitors from Jerusalem were the same "men from James" who had also disrupted Antioch (2:12). As for Titus, however, we have no cause to suppose that Paul's gentile colleague was himself comparably famous among early Christians in Asia. To the contrary, Paul's acknowledgement that Titus's Gentile status had recently been at issue in Jersualem provides a strong reason to expect the opposite: that Titus was not generally famous among early Christians. This lack of evident fame also refutes the suggestion of F. F. Bruce that Paul was specifically refuting a rumor about Titus previously spread by the 'Judaizers'; if Titus was not famously well known at this point then Bruce's scenario should require Paul to explicitly clarify that he meant the same Titus about whom there were rumors. As noted above, "with me" and "Greek" do not specify one uniquely identifiable person.
Logically, the above considerations leave us with only two possible reasons why Galatians in four different cities might be expected to know who Titus was without requiring an introduction: either Paul knew that Titus had visited previously, or Paul knew that Titus was visiting concurrently. But if Paul believed Titus was not currently present then we would still expect Paul to clarify which "Titus" (of all the Tituses on earth) was the Titus about whom Paul was speaking. Therefore, the most likely reason, by far, for Paul to avoid identifying this Titus, is because Paul had sent this Titus to Galatia with this letter.
If we consider the scenario in depth, we find additional reasons to support not just plausibility but strong likelihood. For one, sending Titus offered Paul the strategic advantage of refuting his troublesome opponents by sending an eyewitness who could testify on at least some of the events being narrated in the letter. Instead of merely writing that Titus had met with the "pillars" in Jerusalem and yet remained uncircumcised, Paul could send the Titus himself to corroborate Paul's account. Beyond that, the Galatians could ask Titus questions -about Paul, about his visit to the holy city, about whom he met there - and if any man in Galatia remained skeptical about Paul's claims, then Titus had the option to reveal physical evidence.
Finally, supposing this strategic scenario provides a practical purpose for Paul's two ancillary descriptions, imbuing them with additional meaning. First, the phrase "who was with me" now conveys an implicit challenge. I'm telling you what happened, and he was there too. If you don't believe me, ask him. Second, the phrase "being Greek" denotes an ethnic origin that is not merely non-Jewish; for instance, a Samaritan who became Christian would not be "compelled to be circumcised" because Samaritans still considered themselves children of Abraham (cf. Acts 8:5-39). In this light, the most specific denotation in Galatians about Titus is the fact that Titus (like all Greeks) had not been circumcised in his life before turning to Christ. Taken thusly, "being Greek" is not merely Paul adding a detail but emphatically suggesting there is further proof here if you absolutely need it.
It is further likely, in my opinion, that Titus and Luke carried Paul's letter through Galatia together, and then kept going west to their pre-arranged rendezvous point, at a city they could not fail to locate because it stood next to the ruins of the most famous city in Asia Minor ("Troy"), and this would then explain why Paul found Luke in Troas after leaving Galatia and not feeling able to linger near Ephesus.
But now I have reeeealllly digressed.