You can get tone generators and if you get software to generate your own waveforms, just normal playback is effectively an arb gen. It's plenty capable for what it is, but you'll be missing a bunch of things over a standard sig gen sort of by definition: limited ranges (no ranging, generally), limited output load settings, no calibrated output level (and it will vary with loads because they're not designed to try to), limited output bandwidth, and with default software, the waveforms available are going to be just sound files or very basic tone generation (though if you find the right stuff or the right synth, it's certainly all possible).
For some applications, a sound card may do just as well if not better, but a proper function generator is going to have a lot more flexible output and will have an interface better suited to what's typical of electronics work.