Generally, where there's a 30A circuit, a 15A circuit is also available, but there are exceptions. You may have wired a RV hookup with only a 30A receptacle, but you want to run some lights, or tools or ? with a 15A plug from that on a temporary basis. The circuit is (presumably) good for 30A, so adapting to a lower amp receptacle is reasonable --- it won't hurt anything to draw fewer amps through the circuit.

Adapting from a 15A socket to a 30A socket can be dangerous; the 15A or 20A[1] breaker or fuse on the circuit won't immediately open with a 30A draw, and the wiring will heat up during the time it takes for the circuit protection to open; possibly long enough to cause a fire. Of course, just because a load has a 30A plug doesn't mean it draws 30A all the time, there are conditions where using such an adapter is safe, but it requires knowledge of the load.

[1] US NEC code allows for a 20A breaker on circuits served by 15A receptacle, as long as there is more than one receptacle