"Workplace psychology" is the fastest growing branch of psychology? Why is this?

I think in the workplace must be born a new figure: " an investigatorhuman psychology " (sorry I do not know if I can express myself well in English .) One who is also a sociologist and psychologist who understands the practical needs of the workers.It can not be based only on the study must get to the level of workers, investigate their needs and helping them to reap more profit . A happy person to work makes it40 % more. There needs to be more talk , contact with employers , understand their expectations , make them part of the work process . Starting from a small working up to higher positions , each with their own life and their own needs.A good leader listens and helps its employees and may make use of a person who is not only based on Freud but who can really understand and investigate every single life and only one person.

Because of the large and the strong relationship between the job satisfactions/high level of motivation along with the smooth work and the high level of sales and service.

Workplave psychology is a key HR topic dealing with people in the workplace, and it covers the complete lifecycle of engaging the employees in the workplace.

I take this topic under the "Culture" dimension within my own "MV-BTC" framework which is Millennia-Vision's framework for projects and services targeting the business, technology and cultural aspects associated with large/complex projects and/or key/strategic initiatives.

Some ot the headlines under this topic include ..

1. Employee selection process ... vs... job description and work requirements
2. Assessing "hidden" aspects of the employees like "attitude", "discipline" and "personal maturity" dependent on the special needs of the job like "working under continuous pressures" or "holding responsibility/liability towards other employees, customers, or company assets"
3. Work motivaton and attitude assessment
4. Enabling the employee to do the assigned wotk through training and skills development
5. Assessment of the attitude and discipline at work in real life
6. Taking corrective actions in case of deviaton

This is just a sample that goes under the "culture" dimension, fits within HR and is covered by my MV-BTC framework !!

Hope this answers your question, if you need further help, let me know !

it is very important for a healthy work environment, this includes the mental health of coworkers. 

Workplace psychology is important because it affects the labor or employee satisfaction inside the organization and it reflect the degree of absentism and turnover which affect the internal environment positively or negatively.