Use Cases for an External ADC
If the internal ADC of your microcontroller performs the job you need it to then no, there is no need for external ADCs. But then, that's not who they're aimed at.
You have covered most of the reasons for an external ADC, but there are a few more, and in my opinion, they are some of the most important reasons:
- You need a different sampling technology - for instance the internal ADC is SAR, but you need to do Delta Sigma.
- The internal ADC, because it is internal, and shares the same die as the main MCU, will never be 100% free from the noise of the rest of the MCU, so an external one would be possible to make ultra low-noise
- Your microcontroller / SoC / FPGA of choice has no ADC. The latter two are most likely - most common SoCs and FPGAs don't have any ADC at all. Yes, you can get ones that do, but many don't. So you add an external one.
For point 3, take the Raspberry Pi for example. That has no ADC available at all, you have to add an external one to do any analog work at all.
Another few reasons to favor an external ADC:
Many external ADC parts include differential inputs, while microcontrollers' built-in converters often don't. In cases where inputs have a lot of common-mode noise, that can be very important.
Many external ADC parts include an amplifier stage before the converter itself, thus allowing the converter to measure a high-impedance signal directly. On many microcontrollers, the act of sampling an input signal may disturb it. Depending upon the nature of the input signal, this may vastly increase the acquisition time necessary to make accurate measurements.
Even if an internal ADC is twelve bits wide, that generally won't mean it takes readings accurate to one part in 4096. A typical external ADC will often have better specifications than an internal one, even when both have the same advertised bit depth.
Integrating an ADC onto a microcontroller is easy. Integrating a good ADC is much harder. Guess which is more common.
Another reason for some external ADCs to exist : they have been around quite a lot longer than micros with internal ADCs, and designed into many products. Possibly 20 or 30 years longer. (Probably not the case for the SOIC part linked, though it may be a modernised die-shrunk variation of a classic part)
Where the ADC doesn't have stellar resolution, accuracy or speed, yet commands a premium price, this may be the reason.
Even for new designs it may be preferable to re-use blocks that work well, rather than re-engineer around a newer part (even if the resulting integration reduces BOM cost). That reengineering can be expensive; the test and regulatory approvals process, even more so.
Now if you're starting from scratch, and your chosen microcontroller has enough ADC channels that fulfil your requirements, none of the above applies.