When looking over a field, especially just before the ploughing period or just after extended rainfalls, one can easily be discouraged at the task of clearing the field or weeding. However the best way to address the overwhelming task is to simply start. Once one starts the task will appear easier and they may even see opportunities for making the work lighter. Hence our ancestors were using this as means to encourage us to not be paralyzed by the magnitude of the task but rather to start somewhere and build from there. In other words, to succeed, one must begin the task and not just focus on the end result because they can change plan or strategy along the way in accordance to prevailing circumstances.
The proverb is used as an encouragement when one is confronting a task that seems insurmountable, by advising that once started it will be all the more easier. It is used when someone feels so overwhelmed by a task and seems like they do not have a starting point and it compels them to simply start!