What's the advantage/disadvantage of oversampling followed by decimation, verses sampling at the correct rate to begin with?
I may have originally misinterpreted your question so I'll change my answer here.

As to why we don't typically just sample at our a decimation rate (i.e. the Nyquist rate for the bandwidth of interest). It really comes down to the practicality:

If you had a 200hz signal of interest, yes you could sample at 400hz assuming there was absolute no noise in your system. However, there will be noise and you would need an anti-alias filter to block all frequencies greater than 200hz. The issue with this is that many anti-aliasing filters are slow roll-off because they're cheaper and easier to implement. Higher order analog filters are simply more expensive and difficult to implement (they're more sensitive to component tolerances).

Instead, it would make more sense for us to over-sample our signal such that we could use a lower order anti-alias filter.

Let's say your signal of interest is still 200hz and you decide to sample at 10khz this time. You put in an anti-alias filter at say 5kHz with a slow roll off. This would prevent aliasing frequencies of getting into your digital signal while being able to use a cheap/easy anti-alias filter. You would then perform your decimation operation. Note that the decimation operation includes a digital low-pass filter such that aliasing doesn't occur in the down-sampling process.

Summary: the further we separate our bandwidth of interest from the Nyquist rate, we can use lower-order anti-alias filters which are cheaper to implement.