The reason for including signets is that they ramp and fix mana, which allows for more colors per deck and more interesting draft pick choices.
The reason for including trilands is that they fix mana but are not an auto include. They can also be used as duals in two color decks so they're never really useless.
Most competitive draft formats are composed of three major archetypes: Aggro, Control and Ramp. Therefore, these are the main archetypes for this draft, with some minor archetypes contained within (like RedWhite Tokens for Aggro).
