An amplifier takes a low level line signal and amplifies it to drive a transducer, whether that happens to be a headphone or speaker.
Even a smartphone has a headphone amplifier, it just happens to be integrated into the audio chip which in Snapdragon phones is in turn integrated in the CPU's chipset. Think of it like how games need a 3D graphics accelerator, and smartphone audio chips are like CPUs or APUs that range from something like an old Intel i5 with an HD420 GPU to a current Ryzen 7 4700U with Vega 12 graphics. The problem there is when your game ie headphone can't run at 60fps minimum on anything less than an RTX 2070 Super, except in audio there isn't an easily quantifiable metric like fps. All you get is "if you can deliver this much power into this impedance load, and this load has this sensitivity, there is absolutely no chance in hell you will clip the signal unless you try to make the softer parts of a classical music recording
Also, "easy to drive" is kind of as compicated as "what hardware gets xxfps on this game." On the latter it's a matter of how detailed the graphics are, how much processing power does it need between the CPU (strategy games) vs the GPU (fps and similar), or whether it uses a more efficient API like Vulcan. And then it can happen that low cooling capacity can throttle or prevent the CPU and GPU from ramping up so despite the same CPU, RAM, and GPU+VRAM one properly built system can be 3% faster while running 12dB quieter. On headphones, it's not just a matter of "how much power for given sensitivity." Power delivery changes based on the impedance, and so does damping factor. If your amp circuit's - regardless of whether it's a separate amp or built into a smartphone or whatever - output impedance is high enough and the headphone's nominal impedance is low enough, you're going to end up with the amp having less control over driver movement.
I'm still lost at what "converter" is. Convert what? Like a phono preamp? I seriously doubt a TT that for some reason has BT for convenience (despite negating the whole point of using vinyl) would then be inconvenient enough to make the user figure out whether it has an MM or MC cartridge and ergo buy a phono preamp that works with either one.
