The answer to this question is a resounding … Sometimes.
Basically, there are a couple of things we need clarification on before we can answer this properly.
What Do You Want to Do?
The need for an audio interface will depend entirely on what you want to do, and how you want to do it. It will allow you to use professional grade studio monitors. Whether you need that level of sophistication will hinge largely on the task at hand.
Composing – You don't need an audio interface for composing music. You don't have the need for additional ins or outs. You can save the expense at this point in the process.
Mixing – This gets into a gray area. If you're using studio monitors, you may decide to use an audio interface for a cleaner, true sound mix. You could benefit from using an audio interface, but it's not a necessity.
Mastering – Here is where I'm going to say yes, an audio interface is your best friend when mastering. You need to listen in specific detail that is only going to be possible while using an audio interface.
What Kind of Music Are We Talking About?
Is it just you playing guitar? Are you working on a Hip Hop song? Would you like to record a friend singing the latest song she wrote? You don't need the multiple inputs from an audio interface.
A couple people … a trio? Maybe we want one, but nothing large scale is needed.
Do you want to record a full rock band? Let's think big … how about an orchestra? You're going to need the additional capacity of a significant audio interface.
As illustrated above, certain genres of music require enhanced capabilities, and you'll want to have an interface to help with clarity. Other genres can be quite simple to record and enjoy without all the bells and whistles.
What Level Are You Working On?
Are you enjoying music on a hobbyist level? Do you like getting together with a couple friends and jamming? Maybe you just like sampling beats and playing around with music. You probably don't need an interface at this level.
Are you wanting to record your bands practice sessions before a show or recording? You might want to call in some help from an audio interface, but it's no big deal. It's still not pro level stuff.
Are you starting a music studio, where you intend to record and produce all kinds of bands? Do you want to be prepared for whatever walks in the door? Have an audio interface and know how to use it. For commercial reasons such as these, you probably should go with a high-end interface.
How Much Should You Spend?
If you're the type of person that always has to have the best equipment, you've probably already made up your mind that you need an audio interface … You might have even decided to spend top dollar for it.
If you're still trying to decide, the driver here is simple. How many inputs / outputs do you want to have? The more ins and outs you want, the more you're going to pay. You can get something simple for around $ 100 – $ 200, or go as wild as you want to.
The general consensus is that you should keep your audio interface as simple as it needs to be, while investing more in your monitors themselves.
Will You Notice a Big Difference in Sound?
That really depends on the sound card in your PC. There's been a great improvement in sound cards over the last couple years. A high-end sound card vs a low-end audio interface will probably yield the same sound quality, all things being equal.
On the other hand, if your sound card is not very good, and you go with a high-end interface, and high end studio monitors, you will undoubtedly notice a stunning difference.
Does This Help?
I know you were probably looking for a yes or no answer, but it's definitely more of an essay question. You can find people who are on both sides of the argument, but I think most scenarios fall in the middle somewhere.
Audio equipment is expensive, so it's good to ask the question and give some thought to what you'll be doing before diving in and spending a lot of money.
I hope this has given you some food for thought. Have fun on your musical journey … with or without the interface!