Running a website costs time and money. Neither domain nor hosting comes for free and producing daily fresh content can easily eat up an entire afternoon. A well researched article may even take up several days or even weeks to write.
Not everyone can afford to be altruistic about this and the wish to cover at least the running costs is quite understandable.
The problem with earning money from a website is, that websites primarily deal in information and information does not have a direct physical manifestation. Most people instantly understand that tangible goods can only be acquired by taking them away from the original owner, who usually expects some kind of compensation for transferring the ownership. Information is different. Information cannot be owned, it can only be known and transferring knowledge does not imply taking it away from the original source.
Our culture has long since established the rule, that that what is taken away must also be paid for. However, since knowledge is always duplicated when acquired, this rule does not apply and people fail to understand the necessity of payment.
For the past couple of decades, content providers have tried to work around the problem by binding information to a physical medium (books, magazines, audio tapes, etc.). That is, trying to make the real product ride piggyback on a physical object, giving people once again the impression of trading according to the age old social contract.
Using physical media as an adapter between an old business model and a new kind of commodity worked well for the time being, but never really addressed the underlying problem. A fact, that is increasingly getting back at the content industry as technology advances.
With the growing popularity of the internet, more and more of the adapter technologies are becoming more and more obsolete, resulting in a diminishing profit margin of the providers in question and leading to the conclusion, that trying to sell information directly to the consumer is a dying business model.
It is, of course, still possible to make money on the web by selling subscriptions to premium content, but for most blogs, alternatives are required which usually means some form of advertisement.