They are also a common source for mystical/supernatural power - even if they aren't innately capable of supernatural powers.
For instance, the spell that locked away Klaus ' true hybrid powers used the Petrova family line of doppelgangers as a "key" to breaking the curse.
Origins of the Salvatore and Petrova Doppelganger Line
Tessa and Silas were to be married and drink an immortality potion together. This immortality potion was what shifted the balance of nature. However Silas left Tess at the alter for her mistress - Amara. Silas and Amara both took the immortality potion, starting the line of doppelgangers for both of them. Legend has it that due to this anomaly, the Petrova and Salvatore doppelgangers are destined to be together.
Known Doppelgangers on NI
The Petrovas - Amara(the original doppelganger), Tatia Petrova, Katherine Pierce, and Elena Gilbert.
The Becketts - Rose Beckett (the original doppelganger) and Bela Talbot. Rose looked for a way to kill Klaus and consulted a demon for a way out from running away from Klaus. The demon lied and claimed that creating another doppelganger line to unseal the curse for Klaus and would be Rose's way out. He made Rose the first line of Beckett doppelgangers, but for his own gains. It turned out that Rose continued to run from Klaus until her death. The demon then used Bela for her own gains.
The Salvatores - Silas (the Original doppelganger) and Stefan Salvatore.