The Make client column mandatory on the timesheet is related to the option Allow projects with multiple clients – Bucket in the Projects section in the System Preferences page and is grayed out when enabled. The reason being, when bucket method projects are used on timesheet, selecting a client will be mandatory, so that the selected client is billed at 100% for the time entered towards the project.
In a situation when bucket method is not used, (in other words, single client or cost allocation) implies if Client column can have 'All'.
In Cost -Allocation method – Client set to <All> means that multiple clients exists to the project and the cost is shared amongst client with the percentage specified in the project set up.
In a single client method, Client can be set to <All> as application intuitively knows the client name associated with the project.
Therefore, in a single client method or cost allocation method, user's timesheet can either show the Client Name with < All > association next to Project. This is not true in case of Bucket method projects.