Why sample at a certain frequency just to immediately downsample it?
if they wanted the sampling frequency to be 10Hz, why did they not just sample at 10Hz initially?
In order to avoid aliasing, the signal has to be lowpass-filtered before sampling. No frequencies above Fs/2 should be present in the analog signal (or, realistically, they should be attenuated enough to be buried in the noise, or to a level low enough to meet the specifications you want).
If you sample at Fs=10Hz and want to acquire say, 4Hz signals, your filter will need to let them through, yet provide strong attenuation above 5Hz, so it will need a flat transfer function in the passband, then a steep fall-off after the cutoff frequency.
These high-order filters are difficult and expensive to implement in the analog domain, but very simple to do in the digital domain. Digital filters are also very accurate, the cutoff frequency does not depend on the tolerance of capacitors for example.
Thus, it is much cheaper to use a low-order analog lowpass, oversample by a large factor, then use a sharp digital filter to downsample to the final sample rate you actually want.
The same digital hardware can be used for several channels too. At this low sampling frequency, the computing power requirements are very low, and a modern microcontroller will easily implement many channels of digital filtering at a very cheap price.
You mentioned the word magnetometers. This expands the scope a little.
Magnetometers for those not familiar measure magnetic flux and create a proportional output voltage/ signal according to the flux.
It is likely you will also detect a high amount of unwanted "electrical energy", due to the radiated magnetic energy from any electrical cables around.
In fact, directly sampling at 10hz in the presence of 50hz could drive you mad, as you might not be exactly 10hz, and you will see what looks like a slow DC shift up and down over a period of several seconds.
The 100hz becomes significant in helping to null out this unwanted signal from what you actually want to see. This is typical for places where 50hz is found, in the US 60hz of course.
If you are using magnetometers in some countries, the 100hz/ 10hz does not work so well; you might find a different model for these markets.
The answers on antialiasing/ filtering etc are still correct; this is just more specific for your use case.
They don't immediately downsample. They "filter and down sample". Presumably the filter is a low-pass that eliminates aliases that might occur in the downsampled signal. The filtering also might reduce noise by using information from several of the 100 Sps samples to contribute to determining each of the sample values in the decimated (10 Sps) signal.