Almost laughing too hard to reply, but had to offer up some props to Tarekith since he just mastered my new ambient/drone album.
Do you need a professional mastering engineer to finish your work? Absolutely not. With a few basic tools, time and patience you can do a respectable job on your own. I've mastered every one of my albums prior to this one and they turned out sounding just fine.
That being said, I learned a fair bit about the mastering process doing it myself, and there are few real benefits to having a professional do the work.
1) Good sounding room & gear. The biggie. Mastering in a dodgy room or with shitty speakers can be done, but it is a royal PITA. Plus what makes a decent mixing setup does not necessarily make a decent mastering setup. Count on endless revisions burned to CD and carted around to various stereo systems to get a real world reference check. You'll spend a lot of time on this.
2) Having and understanding the proper software. Can you master an album with Waves Native Power Pack or something similar? Yep. But any software has a learning curve. If you're not an expert on your mastering platform be prepared to spend a lot of time getting intimately familiar with it.
3) Understanding the mastering process. Basics are getting track-to-track EQ and levels balanced so the songs flow from track to track. Then comes correcting issues which were not noticed or addressed in mixing, such as clicks (ahem) or timbral problems. Bonuses are getting the songs to sound similar to others in the same genre. Extra bonuses are adding creative touches to the overall sound of the album. Each of these take proportionately greater amounts of time for someone who does not do them on a regular basis.
So far, the only thing that really separates the amateur from the professional is time. Lots of time. What we can do in 2 days a professional can do in 2 hours. Same principle applies to car maintenance and hanging drywall. How much is your time worth to you?
4) Objectivity. This one is a little different, and it's the main reason I decided to have someone else master this album. How many times have you listened to your songs by the time the mixes are finally done? Can you really listen to them objectively? In my case I knew the mixes so well that any little change sounded "wrong", just because it was different. A second set of ears can help with the big picture, and my case that was worth spending some dough on.
Can friends provide the same objectivity? Sure, but can their input be quickly translated into changes that in turn translate to the real world well? See #1-3 above.
In the end, could I have done as good a job as Takerith did? Probably, but it would have taken me a hell of a lot longer and I wouldn't have had the added perspective of someone who has mastered who-knows-how-many tracks. Plus, given our different equipment and experience my approach would have certainly differed from his. So regardless of whether the results were "better" or "worse" they definitely would not have sounded the same. For an album which I created & mixed solo getting someone else's take on the mastering really took the album to a level that I would not have reached on my own.
Or maybe I'm just rationalizing my own capitalist victimization...
Here's a few tips and tricks for mixing and mastering that might clear up the process if you want to try it yourself though: