When you create a work item from a template and assign all tasks for a particular job role placeholder to a colleague, that job role placeholder will be removed from the work team and be replaced by your colleague. The colleague will also inherit any time and budget estimated for that job role placeholder.
If you decide not to assign the placeholder roles to a team member, the job role will be put on the work team, along with any estimated time and/or budget for it.
Later, if you reassign just one of several of the placeholder's tasks to a team member, that team member will be added to the work team, but the placeholder will also remain on the team along with all of the time estimate for it.
Finally, if you reassign the last of the placeholder's tasks to a team member, that team member will be added to the work team and inherit the placeholder's time. The placeholder will be removed from the work team entirely.