If you run a blog all sorts of people visit it. Most do so because they are interested in the web site, and if they have something relevant to say their comments are encouraged - although in some cases the comment will be transferred to the relevant page of main site - with due acknowledgement.
However there are a lot of people who prowl the web and visit blogs such as mine with a view to advancing their own agenda, and it is to protect people like you, who are reading this blog, that all comments on this site are moderated.
I often get comments which praise the blog - saying how wonderful it is, and how useful the writer has found it.. The message almost never includes anything which shows that the writer of the message has read enough to know that blog is about genealogy or Hertfordshire. In addition their text never contains anything useful about the post they are commenting. The commentators assume that I am a vain ignoramus who likes being flattered, and who will be so pleased to receive "friendly" messages that I will throw common sense out of the window. The common factor to such messages is that they always includes a direct link to an external web site (perhaps disguised as a signature). It is obvious that the comment was only made because the writer wants people to click on the link.
Any links sent in this way to me (or as part of requests for information on the real site) are always checked out by me to see who registered the URL, whether the site is known to my security software, etc. Some link to sites which are clearly "dangerous", others to URLs only registered a couple of days earlier (presumably for nefarious purposes?) while yet others are to sites which are trying to sell you something - on-line gambling and porn are commonest - while some may be newcomers to the spamming game that just want to drum up web traffic to try and make their site more visible to Google. All such comments go straight into the spam bin for your protection.
If you have your own blog you may get other visitors. About 10% of the visitors to my blog come from unlikely sources - for example I don't believe so many people living in Russia have ancestors born in Hertfordshire. It is another type of spam designed to artificially increase web traffic to the site that contacted you. The rule is that if you find someone visited your blog from a unlikely looking URL it may well be an attempt to get you to visit that URL - so take care.