Work Instructions promote consistency. Consistency is a basic premise of controlling a process both in terms of internal factors such as manufacturing cost and external factors such presentation to the customer. If all units aren't built identically and consistently, quality suffers.
Work instructions provide visual details on how specific tasks or operations are to be performed. The need for work instructions can be based upon the complexity or criticality of the task and the qualification and competency of the workforce.
It is always preferred to write the work instruction in the local languages as people working are used to this language and feel comfortable with it.
Work instructions become more important the bigger the company is. You have fluctuation in departments and expertise of people is always just temporarily available, because they move to a different position. The more you have documented the easier it is to familiarize with the new area of responsibility.
But don't be too detailed. The level of detail must be reasonable, because otherwise nobody will read the work instructions anymore. And a risk is (at least in our company) that you have lot's of work instructions, similar to each other and developed in parallel. These instructions have to be identified and harmonized, to reduce maintenance effort for instructions.