When you use free services like Blogger.com you can choose to have your blog's URL be hosted as a subdomain on blogspot.com or you can choose to purchase you're own domain. That way your blog's url will look more like money howtoNOTmakemoneyonline.com instead of someblog.blogspot.com.
From what I've seen, there doesn't seem to be much of a difference in terms of traffic when you have your own domain name or you don't. It seems like more of a vanity issue. I've listed the top 30 Blogger blogs hosted using a blogspot.com subdomain below.
If you're blog starts becoming popular and you want to spend a little on it you can get cheap domain names for under $7 a year with free WhoisGuard privacy protection for the first year at NameCheap (affiliate link). I've been using NameCheap exclusively for the past few years and have been very happy with their service and prices.
The Alexa Ranks for the following blogs range from 6,070 to 69,766. They don't seem to be hurting by not having their own domain name.
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