Maintaining the flow.
If you are a farmer you have to buy machinery to be able to plow the paddocks, Plant the fields and harvest the crops, and a lot of other implements associated to the running of the property. None of these machines will constantly run forever without maintenance on a regular basis.
The company that made the harvester or tractor and so on will be relying on you to make shore that all maintenance is carried out at the right times otherwise the machine will not be worth the thousands of dollars that you paid for it.
We as humans need food just as a tractor needs fuel, neither will get far in time without it. A house will eventually fall down if it is not maintained in some way, wind, water and the weather will eventually find a way to bring the uninhabited down.
When building a business there are lots of things you have to do to keep your business up to date as Time is change and the world does not stop turning and you have to always expect competition. Competition drives us to improve what we have already created, to always improve the next step.
If you are building an online business or website you will be constantly finding ways to improve your output by always finding different ways to attract customers to your site. Advertising of all kinds has to be maintained to attract the consumers senses.