Resonant Frequency of LC Circuit in Switching Converter
Let's consider a synchronous buck converter. It produces a square wave of (usually) fixed frequency and variable duty cycle to compensate against changes of supply voltage and changes in load current.

On it's output is a series inductor feeding a capacitor tied to ground. These components form a low pass filter. Let's say the inductor is 10 uH and the capacitor is 22 uF. Let's also say there is no-load (or just a light-load) on the output of the switcher.

If you did a bode plot of the filter you would see this: -



What if the switching mechanism where aligned to 10 kHz (instead of say 300 kHz)? Answer you'd get a massive sine wave superimposed on top of the DC output that the switcher (and load circuit) is not equipped to cope with. You'd also get substantial dc current taken by the switcher because, at resonance the LC network acts as a short with only the R part offering a non-zero impedance of 0.1 ohm on this example.

If the load current was larger you'd find that the Q of the LC low pass filter would substantially reduce and this could be livable-with but there is no significant benefit designing the filter frequency to match the switching frequency.

Here is the site that the LCR calculator was on