Recently, I've been thinking about their purpose. It is currently a HUGE mystery, which is getting to me. Honestly, here is my theory:
So Nina Dobrev revealed back in November (TVD'S 100 Episode Party) that either Tatia or the first Petrova doppelganger was being introduced/alluded to later in Season 5. I think that Tatia or the 1st Petrova doppelganger was a member of the Travellers and that she is needed for something huge. Tatia or the 1st Petrova was most likely a powerful Taveller witch who had an agenda but was stopped when they were killed. So the modern Travellers have gained interest in the plan Tatia or the 1st doppelganger were trying to do. Also, Stefan and Elena's blood was used becuase the doppelganger blood was a binding property in the ritual/spell. Maybe Tatia or the 1st doppelganger was trying to create a new type of vampire (faster,stronger,smarter) but they were killed before they could accomplish it. And then that is where Dr. Wes comes in. Using his knowledge of science and they knowledge of magic could finally create that vampire.
BUT WAIT, THERE'S MORE! The Travellers will realize that they don't know the other binding potent for the spell, so they will have to revive Tatia or the 1st Petrova doppelganger to access it. But she will then betray them and her ultimate agenda will lead into Season 6.
Just a theory! Please show some flaws or some ideas. All ideas are welcome :)