Deities as erotic couples symbolized the ritualized union of opposites in Vajrayana doctrine, primarily wisdom and skillful means (so two-in-one).
In conjunction with these meditative techniques, one used tantras (magical texts) to summon the deity.
The invocation of deities in Vajrayana borrowed much from Bon, which was the native religion of Tibet. Bon put much attention on placating and seeking boons from various ferocious deities.
Eventually, after Vajrayana came to Tibet, Vajrayana and Bon started to share deities and ritual practices for contacting them.