Firstly, there are many Inca Trails, and secondly, there are many more Inca sites than just Machu Picchu. The whole area around Cusco and the Sacred Valley is covered in trails and Inca sites just like today's tarmac roads connect each modern-day town. Cusco was the heart (more often described as the naval) of the Incan Empire that stretched from Quito in Ecuador, down through Peru, and as far south as Santiago in Chile, covering an area similar in size to Western Europe.
Apparently many more Inca sites and trails can be found all over these parts, but there is a concentration around Cusco on the centre of the Empire. Cusco was the primary capital and this was where the Incan ceremonies and festivals were held, during which mummified Incan rulers or nobles that had transcended in the 'higher world' were carried in procession around the central square. The primary festivals were the summer and winter solstices that signified a time of change that the civilisation relied upon for farming, house building, and organising their society.
Incan society included scientists and astronomers that lived amongst the priests, nobles, and rulers, in citadels like Machu Picchu that were built high in the mountains as near to the 'higher world' as possible. The 'higher world' (represented by the highest flying bird in the world -the condor) referred to the gods, embodied by the stars that filled the night sky, and it was the appearance or location of certain constellations that provided more detailed guidance for the Incan calendar and subsequent actions in the 'middle world' which was inhabited by mortals. Human sacrifice was ritually practiced as well as (probably much later) sacrifice of Llamas or Alpacas. The sun was also worshipped as it was understood that it brought life to the earth, and many advanced scientific and architectural techniques were used to build sundials and temples (like those atop Machu Picchu) in which shadows were cast with great accuracy to provide the Incan rulers with significant knowledge about their place in the world and consequently hold great power over the common people.
The Incas were described to us as an elite society that had either developed or improved the technology of their time to such an extent that it gave them a significant competitive advantage over the people around them. The Incas were effectively the ruling class of the Quechuas, and seemingly saw themselves as a kind of gods over men, even deforming their skulls to make them appear more scholarly or knowledgeable than ordinary mortals. Many other insights into Incan society can be observed along the trail, especially advanced city planning with channelled water supplies, irrigation for farming on specially constructed terraces, earthquake-proof stone housing built without using mortar, and other special constructions for astronomy and religion. Each site was strategically placed not only for food and water supply but also for control, as the Incas established administration centres to control the movement of people along the trails, as well as managing a system of runners that quickly passed messages (using the Inca system of writing) between sites across the Empire. Each site seems to have had a specific purpose according to its location, which may have been administration, farming, astronomical, sacrificial, political or even just a messenger station. We passed by sites of each type along the trail...