The placeholder attribute specifies a short hint that describes the expected value of an input field/textarea. The short hint is revealed in the field before the user enters a value. It is just a temporary hint and has nothing to do with logical executions in the backend.
The value attribute is used to set the default value to elements. It represents the value related to the input and the content in the value set is sent to the server on form submission. If none of the manual values gets inserted, then the default value will get passed on submission. The value attribute specifies the default value of an input field. If no value is specified,then default one will be taken during form submission.
It is the default value and has much to do with logical executions in the backend. This means, when the value is not specified, then the default one will be taken during form submission.