Burden multipliers in planning phase

Hi all PA experts,

we have a question related to burden multipliers when planning sales pahase budget.

We plan human resources using resource class PEOPLE, which rate already includes burdens.

When adding resources to workplan we receive an error  of missing cost rate.

After adding cost rates we were able to proceed, and no burdens were generated.

We wonder why we have to add these rates at this phase.

Is this requirement coming from any setup?

Pls tell us if you have any ideas of this matter,

Thanks in advance,

User172025