So you just got your first set of decent headphones and need an amp, but you don't know what to get. If it's under $500, and you aren't looking to break the bank, I can't recommend this enough.
Everything that makes sound needs an amp, but why a dedicated amp is probably what you meant. Without being pedantic, this is the part of the audio chain that takes the sound from the very quiet "line level" and brings it up to audible volumes. The amps job is to give your headphones both the voltage, and current they need to not only get loud, but have enough power to control the driver tightly so it can make more than one note at a time. Unless you like hearing only one sound at a time, and not something as complex as music, this becomes important.
Someone is sure to get mad at me for try to dumb it down this far, but to keep it simple, voltage is the part that makes it loud, and amps are what give it the control over the driver. Something like 250ohm Beyerdynamic DT 1990 resist a lot more than more common headphones in the 32ohm or so range, so they need more voltage to get louder. Planar magnetic drivers found in the Hifiman Sundara have a low ohm rating, but really take a lot of current to get the low end really going. Without enough current, you will lose a lot of the low frequencies, and there won't be enough power left to get the details out of the high range while it tries and fails to push the low end sounds.
This is leagues better than things like the Apple dongle, or even most of my integrated amps into laptops, phones ect. The Auteur and Sundara both feel like much of the bass region is alive and bass heavy music is much more listenable than most integrated amps. Bass doesn't feel like it goes on forever, but it's very enjoyable on most tracks, and is a very worthy step up from integrated amps. If you like bass, and you have something that's starting to get into mid-fi, I'd recommend stepping up to some sort of dedicated amp for this range alone.
There's not a ton to say here. I personally didn't hear a ton different on mids, but it's also one of the easiest ranges to get right, at least to my ears. I wouldn't say that it does anything exceptionally correct, but also nothing offensive either.
It turns out that when you don't have a driver that's starved for power flapping around, you can actually get a lot more micro detail out of them. Female vocals come through more clear, stringed instruments get their life back, and you can start to hear even the breath of the singers on some tracks, and not just the words they are singing.
For the cost of $99, I can say that this is one of the top recommended things I recommend to people after they start dabbling into headphones. IEM's being more sensitive means that it will play well with this amp, but headphones can start getting some actual use out of it. Both the Sundara and the Atrium sounded better without question on a dedicated amp to the point that I couldn't recommend them without at least the intention of getting something like the Modi unless you just don't like tracks with any bass in them, and don't mind missing details. Unless you want to step up to something like the Asgard 3, this amp has amazing value, and it even doubles as a preamp so you can control your speakers or headphones with one knob. What more could you want?