It is likely a mixture of anti-muggle born sentiments and an attempt to stop small children from causing damage that can't be easily repaired by their wizard parents before a muggle notices.
(Mostly just a plot device though.)

 

As quoted earlier, it seems to be on the child themselves with an expiration on their 17th birthday.

I'd speculate that the Trace started out as a safety measure implemented by the ministry.

Think about it: Wizard kid casts spell around other wizards? He's got other magical people able to help him almost immediately, or at least within a close vicinity.

If a muggleborn decides to mess around with a spell? No one nearby is going to know a damn thing about how to reverse, say Dudley's pig tail, without surgery.

Then the other laws against under age magic were probably built up around the Trace to prohibit stupid teenager things from happening to muggleborns by adding the whole "breaking the laws" disincentive on top.

That it helps support the statute of secrecy is probably a side benefit in the modern age more than the main reason behind its first casting.

 

I have a pet theory that the Trace is tied to individual wands and reports all magic within a certain radius of the wand, along with the location, back to the Ministry. This seems needlessly convoluted, though it fits with the canon uses of the Trace, iirc.

Edit: This excludes the Voldemort revival and fight at the end of GoF. Hmm.

 

If the Trace is on the wand, and not the wizard, then it should have been possible for Harry to portkey to a safehouse in book 7 while, say, Hagrid drove his wand to meet him. That they went through the trouble of using brooms and other similar modes of transport for Harry implies that they could not reasonably separate him from his Trace.

 

Fair point.

It seems strange, then, that the Ministry doesn't react to unintentional magic made by young muggleborns, as not addressing it should carry a risk of discovery. It's not like wandless magic isn't picked up by the Trace.

 

The Trace is likely not placed upon a magical child until they are of age to attend Hogwarts, as the Ministry is likely incapable of determining the identities of every magical child in a country of over sixty million people. Consider the following passage about the Quill of Acceptance, which apparently decides who goes to Hogwarts:

However much stock you place in Pottermore, I find it likely that the magic used by Hogwarts to find magical children without prior contact is esoteric and lost to the ages, and that the Ministry can no more find every muggleborn wizard or witch than they can create another sorting hat. Given that the discussion of the Trace in Deathly Hallows often takes on the following form:

The Trace, from this conversation (and others like it) must be placed upon a person, and therefore requires prior contact with them. From the first passage - as well as other major plot points throughout Deathly Hallows - we know that the Ministry has significant difficulties tracking magic use without the aid of the Trace, and from the second passage, we see that the Ministry has to actively place the trace upon Wizarding children.

Thus, the logical conclusion is that, prior to a child receiving their Hogwarts letter, the Ministry has not placed the Trace on them because the Ministry did not know they existed. It cannot even do so automatically, for that would require that they be capable of replicating the magic in the aforementioned Quill. As such, when (for example) Ariana did magic in front of muggles on accident, the Quill of Acceptance took notice, but the Ministry did not. And unfortunately, the former did not care quite so much for the Statute of Secrecy as the latter.

 

^this is what I meant by the Trace being location-based magic. A single casting that effects all individuals of a given specification in a given area at all times, as opposed to multiple castings on each individual. Idk, probably because it's much less Orwellian.

In hindsight that's probably not the best term for it though

 

Wait. If the Quill records the first use of magic by every child, why isn't someone from the Ministry checking the Book every day?