Farlanders is not only a city builder, but also a game of numbers.
If you have already completed a few missions, you might have noticed that your "profit margins" are pretty low by default - each colonist consumes 1 food/turn, and a farm produces 5 food/turn while requiring 4 workers. And you can't get very far if ~80% of your population is working at the greenhouses.
That's where adjacency comes into play - by placing a building in favorable conditions, you can get extra 20% or even 35% output from it, which translates into needing fewer buildings, having more resources to spare/trade, and having more workers available for other activities.