Section 230 protects ISPs and websites from civil liability for the stuff that's posted on them, or for removing illegal content. That is to say, Comcast can't be sued for libel or whatever because you wrote that George Bush is a serial cocksucker on your personal blog and someone accessed it over their network, because they aren't the publisher of that information, they just own the infrastructure it is transferred over.
This also prevents ISPs and websites from having to censor or approve all content that is uploaded on or through them. Without this protection, ISPs and websites are liable for everything they host and distribute. It's why moot didn't go to jail because some faggots made terrorist threads on 4chan.
Related to the effort to remove section 230 is a bill called SISEA. https://www.xbiz.com/docs/news/256391_SasseMerkleyDEC2020.pdf
The purpose of this bill is supposedly to put a stop to sexual exploitation via "revenge porn" or coerced porn or whatever. It also essentially seeks to install a new regime of intensified internet moderation and censorship. While the text of the bill seems to be oriented towards pornographic sites, in effect it would require the screening of all visual content uploaded to a website to check if it's a) pornographic and b) no one in it is on their "does not consent" list. It also imposes fines for websites that receive complaints that they're hosting porn against the wishes of the person in that image or whatever if they don't take it down within 2 hours of the complaint being received.
The purpose of both of these is to monitor what is being posted on the internet, by whom, when, and where.
As to why:
The internet has been absolutely integral in disrupting American imperialism over the past 20 years. It was an indispensable tool during the Arab Spring and other protests as a means of coordination and spreading information. It was essential in revealing the US's wrongdoing abroad, as well as the huge disparities in living standards between the US and parts of the EU. It played a major part in the meteoric rise of Bernie Sanders, and among people under 40 is the primary method of both communication and information sharing. It also allows for simultaneous, global communication for virtually nothing. Without the internet and these protections, the revelations brought by Edward Snowden and Wikileaks would not have been possible. The internet disrupted the old corporate domination of mass media and helped to democratize everything from journalism to science to politics. It also threatens the massive media empires in other ways, like facilitating piracy, or providing alternatives to corporate published media.
For these and other reasons, the internet as we have known it all our lives is a massive thorn in the side of the bourgeoisie. They don't like that their secrets keep getting posted on it. They don't like that their property is being shared over it FOR FREE. They don't like that it gives people alternatives to their propaganda. They don't like that the disruptive effect the internet can have on the business scene. From the point of view of the bourgeoisie, there is nothing about the internet that is good for them as things stand now.
It's no real secret that the US is angling to try and get its increasingly restive population under control, as well as start a war with Russia, Iran, and/or China in as near a future as possible. Depending on the metrics you look at, China has already eclipsed the US in several respects and has reached parity in several others. Since WW2, the US has relied on its overwhelming technological superiority and industrial capacity to fight its dominance as its ultima ratio
. The linchpin in this since even before WW2 has been its massive fleet. It relies on its aircraft carriers to project its power around the world. It relies on its nuclear submarines to hold the nuclear dagger to the world's throat.
Now, not only is China reaching parity with the US technologically (and arguably surpassing them industrially), both it and Russia have developed a counter to the US's carriers in the form of hypersonic cruise missiles, to which the US currently has no countermeasure. The US fleet is expensive and aging and their recent moves at updating and replacing them have produced mixed results at best. It takes years and billions of dollars to build a new ACC, while cruise missiles can be mass produced relatively cheaply, and all it takes is one in the right spot to send that expensive ship to the bottom. In short, the window through which the US can be reasonably assured of victory (according to its calculations) is closing faster with every passing day.
While they might seem as unrelated as possible, the US's drive to war and these moves at censoring the internet are directly related. They've spent years building the infrastructure and invested many billions of dollars in monitoring the internet and trying to shape or influence discussion on it. The US doesn't want their citizens to be able to educate themselves with alternate news/propaganda sources, much less Russian, Chinese, or Iranian sources. It also doesn't want them to access information like Hillary Clinton's emails or the Snowden Files. It especially wants to put a stop to websites like /leftypol/ and chapo, or at the very least keep them under the strictest surveillance, especially after last summer's protests/riots.
If I had to guess, that would probably be what freaked the bourgeoisie out the most: seeing the poors out in the street fighting the police and setting up their own autonomous zones. The bourgeoisie want to nip that
right in the fucking bud, and either frustrate any further activity or strictly monitor everyone involved in it.
tl;dr: to control the proletariat and facilitate their drive towards war to maintain their hegemony and the supremacy of the bourgeoisie