People do all kinds of different things on the internet and, in some cases, they are also all kinds of different people. Facebook has a strict policy that you must create an account under your real name in order to use the site, while Twitter does not. In general, most websites allow you to enter whatever name you like and people take advantage of that.
For example, they may be participating in a political forum and espousing their liberal views. Using a pseudonym allows that individual to remain protected, theoretically, and that might prevent them from getting in trouble with a conservative boss at work. Or someone who is thinking of coming out, might seek information from an LBGTQ forum where their identity can remain a secret until they decide they are ready to share their sexual orientation with the world.
These examples are fine, though it must be pointed out that many people abuse the privilege of online anonymity. Some do this for malicious purposes, such as trolling or other forms of harassment. Others do it to indulge in illegal activity, such as downloading pirated movies and TV shows, or other more serious criminal pursuits. You don’t have to be tech savvy to disguise yourself online either: video private networks (VPNs) are subscription services that provide users with the option of IP addresses in multiple countries. Want to watch something on the BBC iPlayer, but don’t live in England? Simply click on a UK IP address provided by your VPN and the BBC does not know the difference.
Governments and corporations take a dim view of such activity. Russia and China announced that VPNs will be banned in those countries by the end of the year. This is unfortunate because it will rob citizens of the chance to speak openly and potentially criticize those authoritarian governments. But, of course, that is just the sort of openness they seek to oppress.