Thrace was a country situated in the actual north part of the Greece, a huge part of Bulgaria and the European part of the Turkey:
What we know of Thrace is relatively simple: Thracians were not writing. Herodotus says that:
5-3 & sub: the Thracians are the largest nation in all the world at least after the Indians... The only gods they worship are Ares, Dionysos and Artemis. Their king however, unlike the rest of the citizens, worship Hermes separately and more frequently than the other gods
Notice Herodotus uses the Greek names of the gods, which does not help to find what could be the votive original gods.
The War of the Peloponnese
In 431BC Athens entered in war against Sparta (the one of 300). Athens looked for a lot of allies, some of them will be precisely those Thracians the two main reason being first they needed allies, and second Thrace was well know for its wealthy silver mines. Here is Thucydides description:
Sitalkes, (...), was now sought as an ally by the Athenians, who desired his aid in the reduction of the Thracian cities and of Perdiccas [king of Macedonia]. Coming to Athens, Nymphrodorus concluded the alliance with Sitalces, made his son Sadocus an Athenian citizen, and promised to finish the war in Thrace by persuading Sitalces to send the Athenians a force of Thracian horse and peltats.
All of that is important to notice than roughly in 450 when Herodotus wrote he called Bendis Artemis, the relationship with Thrace and Athens beginning toward 430. Plato writing around 380 this time mentioned Bendis, if that makes sense.
As a side note, later in the war Thucydides is sent by the Athenians to protect a Thracian silver mine against the Spartan general Brasidas and he is beaten, which will lead to his exile. Without the short sight of those Athenians, Thucydides would probably never have written his war of the Peloponnese!
Bendis in Thrace & Athens
We know little of Bendis in Thrace, Wikipedia is by the way illustrating that by showing pictures of Greek representations, here is another one, totally a Greek style:
I want anyone to see that Wikipedia is totally failing to mention they are referring not to the Thracian Bendis but to the Greek Artemis-Bendis. None of their illustrations are Thracians, none of their citations Thracians, neither they mention that...
Here is Richard Farnell notes of her:
The Thracian Bendis was probably in origin and character the same as the great mother of Phrygia, and was worshiped (...) with the same orgiastic rites(...) we also hear of Bendis as the great goddess of Lemnos to whom human victims were offered... The reasons for this association of the Greek maidenly goddess with the Bendis-Hekate-Brimo the patroness of savage magic and terrifying superstition, may have been some external resemblance of attributes, but also probably, some consanguinity of character. Like Artemis, the Thracian divinity was supposed to ride on bulls; like Artemis, she was a mighty huntress, though her weapon was not the bow, but the spear in each hand.
So basically we see that Bendis was included in the Athenian pantheon of god roughly for political reasons namely the alliance with the Thracians. As a goddess of hunt, dark ritual magic and bulls, Bendis was linked naturally to Hecate-Artemis (note the theme of the moon). Notice that what Plato is talking about, the fact the Athenians was riding with torch in their hands is purely an Artemis ritual.