List building is not JUST about sales, it's about building a relationship. A relationship is extremely vital to anyone's success online.
Now a days, unless your Amazon or eBay or Google, no one is going to trust you right off the bat, and hell, even if you are Amazon, there are still many old-school types that won't even trust you then, LOL.
Having a list enables you to have a no-cost way (no cost for your subscribers) to build a relationship with those who are willing to give you a shot at earning their trust.
Once you've earned someone's trust, and you're offering something they truly need/want, sales will roll in like the rolling thunder. Hell, even if they don't need/want what you're offering, earn enough of their trust and they may still buy from you, even if only to support you and your cause.
There's an old adage which goes something like "It's easier to re-sell to an existing customer than it is to convince a new buyer" (ok ok, I know that's not exactly how it goes, but I'm sure you know what I'm trying to say)... the point is, it's true. It IS 1,000,000,000 times easier to sell something to some who has already purchased from you (and trusts) you, than it is to convince someone who simply stumbled onto your site from a google search.
Building a list is NOT just about building sales. You have to build a relationship first, and having a mailing list makes it so much easier as you can communicate constantly with your subscribers...
Ironically,
@Ron Killian (I believe it is him), sometimes talks about how often you should email your list. (1st, I said ironically because I had to go searching for his name and I just noticed he was the last forum member to post, so, all I should have done was look at the post above, and 2nd, I also see that in his post above, he too speaks of what I am about to say... great minds think alike, eh Ron?)
Proper frequency is an extremely important factor when running a successful mailing list. Email your subscribers once a month and chances are good that they will forget about you as early as your second email to them. Email them 4 times a day, and you'll surely piss them off (unless your list is designed for such a schedule, of course).
The key is to email them often enough that they start KNOWING who you are, but not so often that they get tired of hearing from you.
If you're only intent when emailing them is to blatantly sell them something, less frequency is better. If you're providing real-world value, then emailing them as often as once per day could be very beneficial.
Anyways, getting back to the point, having a list and building a relationship DOES INDEED help increase sales as it enables you to building a following...
Think about it, how often do you go out and buy something because your friend said it was cool? Or your sibling said "it really works"?
When building a relationship with your subscribers, it's pretty much the same thing. I know MANY marketers who could successfully sell "shit piles" to their subscribers and make a KILLING.
Why? Because their subscribers trust them, their subscribers don't hesitate to buy ANYTHING they recommend because they have a relationship, a bond if you will.
Well, once again, I went off on a tangent, so I will stop here.
Hope what I said helps answer your question, if I didn't go too far off topic of course.