Argthtjtr

Many synthesizers (like the Elektron Analog Four) have two oscillators. The Novation Peak has even three oscillators. This seems to be common in subtractive synthesis:

Source: https://de.wikipedia.org/wiki/Subtraktive_Synthese

For what practical purpose do synthesizers have multiple oscillators?

Firstly, a single oscillator will tend to produce rather a subjectively 'thin' and static sound. This isn't always the case (as later stages, such as the filter, or a separate chorus stage, can add warmth and movement), and it isn't always a bad thing - but of course if you want to have a one-oscillator sound on a synth that has more oscillators, you can always decline to use more than one!

We're often told that musical sounds are periodic, and that they have overtones that are integer multiples of the fundamental frequency. This often is true of synthesizer oscillators. However, in nature, this is usually only approximately true - a vibrating string will often have higher overtones whose frequencies are a little higher than those multiples, for example. Many acoustic musical sounds also have multiple oscillating elements at slight pitch offsets - the string section is a classic example. Being able to play multiple oscillators, slightly detuned, is therefore useful for producing these subjectively more natural sounds.

You can also take this detuning further and have oscillators tuned in musical intervals (e.g. a fifth apart.) You could also use mix different shapes of oscillator to produce spectra that couldn't be produced by a single simple oscillator.

Possibly the most interesting uses of multiple audio-rate oscillators is to use one to modulate another at an audio rate - in techniques such as PWM, FM, and oscillator sync. These techniques also do a good job of producing more interesting and animated (though not always very natural!) spectra.

I've assumed here from the picture that you're talking about multiple audio-rate oscillators, that would normally be described as 'VCOs' - you'll probably be aware that synths usually have one or more low frequency oscillators (LFOs) that can also be used for modulation purposes.

I think that the shortest and most precise answer to your question is: to produce timbres that are not possible with a single oscillator, including an entire form of synthesis (FM) that requires at least a second oscillator to modulate the first one.

In a slightly longer answer I would describe the West Coast and East Coast approximations to synthesis, as each one takes advantage of the presence of multiple oscillators in its own way.

• 
  East Coast synthesis, mainly substractive, was often adopted by musicians with a more classical training to replicate or at least resemble the sound of real instruments, such as violins, brass sounds or organs. These real instruments sounds are not easy to emulate with a single oscillator, as in nature sounds are not perfect and often there are sympathetic vibrations and other small artifacts. By means of additional oscillators slightly detuned from each other, a sound can be fattened and modified. This is what is usually called a more natural sound, as in similar to what's in nature. Of course, other obvious musical usages of a second oscillator are possible and frequent in this approximation, such as harmonization: detuning the second oscillator (and the following ones) to a musical interval relative to the root note being played by the main oscillator. The 5th interval is very common, as it doesn't change in most grades of major and minor scales and sounds very rich and pleasant.


• 
  West Coast synthesis, on the contrary, tried to estabilish a whole new musical language not based on the classical heritage. The
  most popular brand of West Coast synthesizers, Buchla, didn't even
  put a "piano" keyboard in their first synthesizers in that regard. In
  this type of synthesis, additional oscillators are used to enrich and
  modulate the main oscillator to create a more complex tone with
  techniques like FM, wavefolding or additive synthesis.

Anyway, and with all due respect, I want to disagree with this affirmation in a comment by @Todd Wilcox:

If you've ever spent a good chunk of time working with a synthesizer
and crafting sounds, you likely found out that there's not much you
can do with only one oscillator.

It definitely depends on which oscillator are we speaking of. In the modular world, there are examples such as Mutable Instruments' Braids, that are chosen to be the only oscillator present in many small Eurorack systems, and the variety of sounds achievable is huge. However, I agree that a hard-wired synthesizer with a single oscillator is pretty limited.

Some links:

https://reverb.com/news/the-basics-of-east-coast-and-west-coast-synthesis
https://en.wikipedia.org/wiki/Buchla_Electronic_Musical_Instruments
https://en.wikipedia.org/wiki/Moog_Music