Why do hackers infect websites?

Most often causing damage to a website under attack is not the goal of infection.
The main goal may be spam mailing or most often insertion of hidden or explicit redirects, which send website users to other websites, let’s call such websites Unwanted.
These Unwanted websites can be:

1. malicious code carriers, that infects the user’s computer;
2. fruad websites, that steal user passwords and other personal information;
3. containing paid-access pornography, or other “valuable” information like “how to lose 30 kilos in two weeks”;
4. any simple websites, that simply want to raise it’s traffic ranking;
5. websites, that needs to be disabled by multiple traffic increase, resulting in server overload – there are so-called DdoS attacks.

In their turn redirects to Unwanted websites can be split into groups further on. For example, unconditional redirect on entering the website, redirect on entering the website from search engines, hidden pop-under windows with Unwanted websites, explicit new windows and tabs with Unwanted websites.
It’s easy to guess how the abovementioed means can bring profit to the people behind such hacker attacks. Consequently, malicious people infect websites purely for commencial benefits.
Language Russian.