If it’s private and readonly, the benefit is that you can’t inadvertently change it from another part of that class after it is initialized. The readonly modifier ensures the field can only be given a value during its initialization or in its class constructor.
If something functionally should not change after initialization, it’s always good practice to use available language constructs to enforce that.
On a related note, C# 9 introduces the init accessor method for properties, which indicates the property value can only be set during object construction, e.g.:
class InitExample
{
private double _seconds;
public double Seconds
{
get { return _seconds; }
init { _seconds = value; }
}
}