What geographic factors influenced the building of transportation networks by the Incas from the 12th-15th century CE?

The Inca wanted to prevent erosion of the soil from the floods and believed construction would help do this

The Inca wanted to unite their large empire, which stretched from mountainous to desert regions

The Inca wanted to unite their large empire, which stretched from tropical to plains regions

The Inca wanted to encourage travel and trade, which formed the backbone of their economy​