How does geomagnetism scientist understand the motion of earth's place?

Historically, the recognition of linear magnetic anomalies (produced bu field reversals as the sea floor was created) that were symmetrical about the mid-Atlantic ridge were important to the identification of sea-floor spreading. This in turn was one of the ideas that led to plate tectonic theory.

More generally though, palaeomagnetism helps in the reconstruction of former plate/continental configuration and tracking of movements through deep time. The angle the Earth's magnetic field intersects the Earth's surface changes with latitude, so by measuring the orientation of remnant magnetism you can get an indication of the latitude at which the remnant magnetism formed, and the apparent direction of the magnetic pole relative to that rock at the time it acquired its magnetism. This allows reconstruction of things called apparent polar wandering curves (essentially where the magnetic poles appear to be through time for a particular crustal block). By comparing these curves, it is possible to determine when particular continents were conjoined and moving together and when they were moving separately.