IEMatch is a great tool. But like any tool, you only use it when you need it. Otherwise, you're just adding something else to the chain (purists can cringe now). This is especially bad when you add resistance, as people who run their amps full tilt (bypassing the resistor in the potentiometer) will agree.
In short, it's all about tradeoffs: in some instances adding IEMatch is very necessary to achieve the best sound, and the tradeoffs worth it. But you don't need one if you don't have an issue. Here are issues that IEMatch is marketed toward:
1. Hiss when using a very sensitive IEM or an amp with too high a noise floor- it will solve the problem, not help make it better, but eradicate it in most instances.
2. Tonal changes because of too high output impedance from your amp or DAP. If the output impedance is sufficiently low, rule of thumb being 1/8th or less than the earphone's impedance, you shouldn't have an issue. Most DAPs aim for a 'near zero' output impedance these days because they're designed for IEM use.
3. Using a DAP that achieves volume attenuation digitally AND does so by way of destructive 'bitstripping'. See next comment below.
Almost all DAPs nowadays perform volume attenuation digitally, but do so by calling the volume function in the DAC chip. Here is where things get complicated. As to how destructive attenuation is to resolution all depends on how the bitstream is sent to the DAC by the software, how the DAC is programmed to 'justify' these bitstreams (LSB or MSB), and the noise floor of the DAC chip itself.
Personally, I think IEMatch should only be used for 1 and 2 above and that #3 is a non issue with today's DAPs. Hope this helps