Why does spam exist?

When spam comments are easily recognizable, why do spammers still leave them?

The answer generally lies in the results other than the human results. Often spam programs are built to try to improve the computerized rankings of the spammer's website or services. By being mentioned in more places, even by spam, it can be hard for automated sorting programs to know the true value of a particular website or service. Spamming helps the spammer confuse these automated services and improve their rank in search engines and on other services.

Some spammers are getting more clever and their spam looks like it was written by a real person. Often they fake being a real user but then link back to a service they are promoting. These spammers don't interact in a real way with the websites they are spamming - they only exist to promote themselves.

Think of spam like advertising - except it's really terrible advertising. Some spammers don't care about crafting a good message, and instead they blast out their spam as far and wide as possible. It's like being able to send a postcard for free. If you could send unlimited postcards for free to anyone all the time, would you bother to make each postcard perfect and unique? These spammers have decided that getting their message the maximum reach is best for them.

Some spam even comes in foreign languages because the computer programs that make the spam can't tell where it is appropriate to post something. So some spam may be in Chinese or Russian characters on an English-speaking website. It's easy to tell that it's spam, but it sure is inconvenient for other users who have to run into these kinds of spam.

In a nutshell: Spam exists to make spammers money.

Sometimes spammers are promoting illegal services that they can't advertise normally, or they are promoting malware that will infect your computer and then they can sell your personal information. The best thing to do about spam is to delete it and block the IP addresses (computers) that it came from.