The rationale behind is to avoid the current trend that for certain grants the average age of the researchers receiving it for the first time keeps increasing over the years (See graph).
For example, for the United States National Institutes of Health, a new investigator is one that does not have a story of previous funding.
Other organizations, such as the Alzheimer's Association, consider a new investigator someone that is less than 10 years past their PhD degree.
Many journals have recognized the essential contributions and achievements of early researchers and highlighted their profiles in different formats.
For instance the Nature research group journal Communications Biology has launched a series for early career scientists, reporting some of the rising stars in all fields of biology throughout the year.