In a world where good software synthesizers are both plentiful and affordable, not everyone needs mixers or hardware synthesizers. But a good quality audio interface is essential to get your sound out to your speaker or headphones - and, if you plan to record with microphones, guitars or hardware synthesizers, getting the sound into your computer.
If you use software or hardware synthesizers, a MIDI interface is necessary for connecting a MIDI keyboard to your computer in order to 'play' the software, or control your external gear.
For convenience, not only are most audio and MIDI interfaces combined into one unit, some are now built directly into analogue and digital mixers - making your studio life even easier.