Data transfer is a completely different concept. The data transfer limit of your website has a very significant impact on your business. Basically, the data transfer limit is how much data you are allowed to transfer in a given amount of time. If you can transfer 100 gigs per month, then as soon as your 100 gigs is hit for the month, your customers will find you unavailable and no matter how much advertising you have in place, you will not be able to sell anything or do any type of business due largely to the fact that you have run out of available data transfer.
While bandwidth, or lack of it, will keep customers from loading your site rapidly, the site will be available if they are patient. But the data transfer limit is a different matter altogether. Once you hit that data transfer limit, you will not be able to do business at all.