For all projects there will only be a limited amount of personnel with the correct skill set to carry out the tasks or work packages. There will be other projects that will drain resource. Priorities may change causing a potential diversion of resource.
While it is very common for individuals to be responsible for more than one task an individual can only work on one task at one time. Hence, in practice many people will have to manage their time between multiple tasks.
People are not at their best 100% of the time. Personal pressures, illness and tiredness are just some reasons why most people will not be able to focus their effort all of the time.
Other obvious reasons will include holidays, sick leave or being on training courses.
In addition, if the working environment is poor, distractions and other aspects can lower efficiency. All materials and procedures will need to be readily to hand to make sure lost time is minimised.
Some tasks, as we have mentioned previously, will be of fixed duration because they are independent of the level of resource attached to it. For example, waiting for concrete to dry, one person writing a report. Even in these circumstances management should look for ways to reduce the duration to a minimum.
Assigning personnel resource to tasks should take into account the above. In practice a manager will be very aware of many of the above issues and will need to maintain flexibility to ensure tasks stay on track. Having personnel to cover particular skill areas is very important.
There are many advantages to assigning a person from the project management team to the tasks in the schedule as opposed to those who actually carry out the task.
In practice, it may not be project management team members who are assigned to tasks but managers at a lower level. However, it is the principle that the manager will be responsible for a list of activities and delegate resource accordingly.