If you don’t have your resources fully utilised (booked and working) they may be taken from you by your senior management to be put on projects which look like they need them (i.e. demanding more resources).
Plan your schedules so that your people are fully occupied. This includes trades crews, you don’t want them running out of work because of late deliveries of equipment etc and then being put on different projects.
If you lose staff resources to other projects it is very difficult to get them back.