Q: Why stick a hand in a heifer that has no problem or signs of calving?
A: Good question. When performing an intervention, the goal is first and foremost to cause no greater harm.
The initial poster indicated that the heifer was overdue, the second poster rightly pointed out that up to 2 weeks after the due date may be normal. However, you have to keep in mind that towards the end of gestation, the longer you wait, the larger your calf will be. Since this is a heifer, you may end up having a tight calving if you wait too long.
1) Sticking your hand inside may actually accelerate thing and provoke labour (if it is around the predicted date of calving, the calf will be sufficiently mature and the intervention should not be a problem).
2) To make sure the calf is alive.
3) To make sure the presentation is normal. If there is an abnormal presentation (can't feel a head, can't feel a leg or there are too many legs) the earlier you know, the better the chances of getting a live calf (or calves).
4) And the most important point is to detect a possible uterine torsion.
In the case of a uterine torsion, the initial outward signs may be normal. Because there is a twist, the blood flow to the uterus is compromised and the more time passes, the lesser the chances of saving either the calf or the dam. Some animals even keep on acting as if everything is fine even though the uterus has become non viable. And unfortunately, in the majority of the cases, by the time the vet is called, the calf is already dead. In some cases, the calf must be extracted by C-section. So the owner ends up with a dead calf, a vet bill and possibly a dam to euthanize if the uterus is non viable.
A vaginal exam is relatively safe (when the hind end is washed with clean water and mild soap) and may save you lots of money.