There are a number of reasons why it is interesting, exciting and possible to do NMR in the Earths Magnetic Field.
1. Cost
Conventional NMR requires a permanent or superconducting magnet to provide the magnetic field. These fields are many orders of magnitude bigger than the Earth's 50 microtesla magnetic field. The NMR signal strength increases with higher field strengths, however producing these large magnetic fields is expensive.
Using the Earth's magnetic field can significantly reduce the cost of NMR.
2. Magnetic field homogeneity
Conventional high field NMR instruments require an extremely uniform magnetic field for polarization of nuclear spins and the detection of the resultant magnetization. It is difficult and expensive to make a magnetic field of the required homogeneity (<< 1 ppm in some cases) and so sample sizes are usually quite small. Magnets also have expensive high order shim systems.
EFNMR works because detection takes place in the highly uniform Earth's magnetic field. This means that large samples can be used - partially compensating for the very low detection field. To improve the initial magnetization, a relatively crude electromagnet with a field about 35 times larger than the Earth's field is used to polarize the sample, however the signal detection takes place in the lower, but highly uniform earth's field.
3. Different NMR properties at lower fields.
Many NMR properties vary with magnetic field strength and as such Earth's field NMR allows for measurements of these properties at low magnetic field strength. Parameters such as T2 relaxation rates are strongly affected by field strength and may be significantly different for the same samples at higher fields. These changes often yield information about the molecular dynamics of the sample under investigation. Earths field NMR may provide data that can be used with data from higher field NMR.