Documenting processes is typically a very good idea. Here are a couple of reasons.
(1) It clarifies what needs to be done in each process and allows you to eliminate unnecessary steps thereby increasing productivity and saving staff time and associated costs
(2) It is useful for training new staff; In fact the process flows can be part of an orientation program
(3) It allows the company to identify time consuming steps that would benefit from automation
(4) When selecting new software, it allows you to test the software in the context of what you actually do rather than the features of the software. In fact, the candidate software company can prove their mettle by showing where they add value by eliminating steps and improving workflow in addition to their features.
(5) If you are in a business that is heavily regulated, this documentation is typically prized and can be used as a sales tool to demonstrate the discipline in the business
As to how often they should be done and reviewed – and for the reasons noted above – I’d recommend that this be treated as living documentation and used regularly when making changes to the way work is performed, software created etc. This effort is only valuable if it becomes part of the company fabric and has a purpose.
I just completed a project where I managed the process flow analysis of a 300 person company and designed a software assessment process for them. I’d be happy to talk with anyone who wishes to discuss this further (firstname.lastname@example.org)