Work expands so as to fill the time available for its completion.
A circular version of Parkinson's Law coined by Jeffrey Klein states:
Bureaucracy expands to accommodate the expanding bureaucracy.
The importance of understanding Parkinson's Law relates to how much of a resource (such as time) you allocate to tasks, departments, etc. Every entity—corporate and otherwise—tries to maintain it's own survival. This is usually accomplished through signals of growth such as increases in budgets and personnel.
For a personal efficiency standpoint, you can sometimes constrain your time and continue to produce the same level of output (e.g., fixed schedule productivity).