People are often confused by the terms interface and preamp, so I thought I would take a moment to explain the two.
An interface (Focusrite Scarlett Solo, Apollo Twin, etc) converts your audio signal to a digital stream for your computer. It also converts the computer audio back to your headphones (or speakers) so you can hear it. A preamp is designed to amplify the relatively low signal from your microphone into something usable. It's that simple!
At the studio I work for (Digital Arts NY), we have a variety of mic preamps in every control room. Each one has at least one Avalon 737sp tube preamp/ compressor/ EQ. They sound as amazing as they should for $3,900 each! Those preamps are then patched into our Pro Tools hardware, which is just a larger version of the sorts of interfaces that you might be using at home. This sort of high-end preamp is amazing- but far from mandatory for home use. For one, we don't even touch the EQ section of these units, because consistency is critical. So that remains off. We DO rely on the compression quite a bit for animation and cartoon work. But even then, it's kept minimal so that we don't interfere with the sound quality too much.
Here's where it gets interesting- and sometimes confusing- for voice actors. Interfaces have preamps in them! When you plug your mic into the XLR input of your Scarlett Solo- you're connecting to the interface's preamp. You turn the input gain up to the level you need just like on any external preamp. While the interface preamps are generally solid state, and have less sophisticated circuitry then their expensive outboard counterparts- that does not by any means make them bad! In fact the overwhelming majority of voice actors I work with have units like the Scarlett Solo, and they sound great (provided the booth and mic are also solid).
If for some reason you're not in love with your built in preamp, you can always purchase an external unit and use it WITH your interface by overriding the built in option.
At the end of the day, it's as simple as: the interface turns your signal into a digital audio stream your computer can use, and the preamp (built in or otherwise) amplifies your mic signal to a level we can hear. The time it takes for your computer to convert analog audio (your mic) to digital audio and back to analogue (for your headphones) is called latency. While it's a small delay in the audio chain (depending on your computer, interface and other variables), it can be hard to listen to. That is why interfaces have added direct monitoring or zero latency monitoring. More on that soon!