How necessary is an USB Audio Interface?

Answer


"Worth the money" is very subjective. Let's instead talk about the various factors you have to consider.

Fitting A into B
Good quality microphones usually have XLR connectors. These have three wires arranged so that any interference picked up in the cable is cancelled out.
Many USB audio interfaces have XLR sockets (but check that the one you choose does).
Your computer's microphone input will be a stereo 3.5mm socket. So if you don't buy a suitable audio interface, you have the issue of physically plugging stuff together.
You can get XLR to 3.5mm cables, such as this one.



These are not all that common, and I see anecdotes on the internet about many of them being incorrectly wired. However, it's potentially an $8 spend that would allow you to make-do without a USB interface.

Power
XLR doesn't only carry a sound signal. Condenser microphones need power, and usually the most convenient way to get power to the mic is phantom power, whereby a small current is carried on the signal lines of the cable.
You can expect the XLR sockets on USB audio interfaces to provide phantom power -- but check the specifications before buying.
Your PC's 3.5mm microphone socket won't provide phantom power. With many microphones, you can put in a battery instead - but it's an extra thing to worry about.

Audio quality
The input of the standard audio interface on a PC is not designed for connoisseurs. After all, most of them don't get used at all, and the rest get used for Skype-type chats. However, whether it's good enough for your needs is entirely subjective.
If you can record music from the line-in input to a quality you consider "good enough", then there's a chance the mic input will also be "good enough". The one extra step is the pre-amplifier in the PC which boosts the signal from mic level to line level - again, this may or may not be good enough for your needs.
You'll always be able to find an audiophile who will insist you need more expensive equipment.
If your built-in sound card does an acceptable job with line-level input, then one possible alternative to a USB audio interface, is to buy a microphone pre-amp:

Connecting it as: mic (XLR)-> pre-amp (phone)-> PC line-in.
This would effectively be replacing the low quality pre-amp in your PC with a high quality pre-amp. A good pre-amp like this costs about a third of the price of a USB interface, though, and handles only one mic. The USB interface will contain a broadly equivalent pre-amp for each input it has.

Latency
Latency is the delay between a sound wave hitting the microphone, and the digital audio stream reaching your computer program. If you're monitoring live, add on the delay between the program writing its output stream to the device, and it reaching your speakers.
Non-music uses of a microphone don't need particularly low latency, so the designers of built-in inputs don't strive for low latency. However when you're layering tracks in a DAW, or listening to the computer's output as you play, it's very significant. Effects range from ragged timing on multitracked recordings, to complete befuddlement as you play!
Better audio interfaces have lower latency in general. They generally also have drivers designed for lower latency. Often the driver configuration includes sliders to affect the buffer length. Larger buffers tolerate overloaded CPUs better, but lead to higher latency.

Control and convenience
Your PC mic input is one socket.
USB audio interfaces like the one you've been recommended, have a number of sockets, each of which your software sees as a separate input. With the right software you can even record from two inputs at the same time (e.g. two singers; a singer and a guitar; two mics, one for the instrument, one for room ambience).
They also tend to have dedicated knobs, controls and displays. Some have built-in compression, to gracefully handle level clipping.
It's much more convenient to adjust input levels with a physical knob on the input device, than to fiddle with sliders in software.
It's much more convenient to have a guitar and a mic both plugged in, than to have to unplug, plug in, adjust levels, each time you switch.

The right microphone
Different microphones are good for different purposes. Make sure you know what you want to use your microphone for, before spending money on it.

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