I'd venture to say that there are security devices in that handbag - a GPS tracker if nothing else but I would not be surprised also if she has a pistol in there - last line of defense sort of thing.
I used to wonder if it was the equivalent of the US "football" which is the device for the nukes which is always with the President. Doubt that now - the UK has a different system for their nukes than the US. But maybe she has some sort of phone which also keeps her in touch with the military or government at all times.
We know she uses the handbag to signal her staff and security people too. Maybe she feared a coup from the Midds!
Thanks. That makes some sense, after all, I don't think is so important than the queen (or Kate, or Sophie or whomever) carries her own lipstick as she could absolutely have someone else carry it for her and it would free her hands por whatever she needs.
In Kate's case it would change her awful hand in crotch pose