Alternative frontends refers to websites/services such as Newpipe or Invidious for YouTube.
This is about specifically frontends for proprietary softwares, things such as odyssey.com is off topic.
Techical details are not to be discussed in depth, nor will those frontends be discussed individually.
If something exists, there must be a reason behind it, even if it does not make any sense. With alternative frontends, the most "normie" response would be to be able to access contents from those centralised sites indirectly, such as, when the access of youtube.com is denied in a place, the person can self host (or sometimes just use a new/unpopular instance) Invidious, and with that way they can access contents within YouTube without using a proxy/VPN/custom DNS.
The other reason is to view content that requires an account. Sonetimes the content owner/website does not want you to see contents without an account/verification, which often times does anything but annoyance. Using such software allow users to view such contents without the need of an account/verification.
The intermediate response would be to enhance user's privacy when viewing contents in them, especially with the rise of tracking technology among big tech. However, there are services that didn't bother with such a thing in mind. Most of the "Instagram viewers" uses the Instagram API, some pf the YouTube frontends simply uses the embed, which is practically no different from viewing those content on the website, except it now tracks you with (mostly) just the content.
And the advanced response would probably be to maximise the freedom of use with such content. Some alternative frontends removes the DRM of the content that the website owner intended. With that, people can download contents as they wish (ignoring all that legal stuff), and modify/use/view/distribute them freely.
The attempt in making viewing such contents simplier/as it should be also means that they are attempts of less bloated/more feature rich services. In other words, they are like a custom plan for the website they are using, for reasons they care about.
One of the obvious reasons is that it does not "just work". Imagine one day you're trying to watch a vtuber stream, and you want to donate to the vtuber because she (or he, as you probably know why) just said something that makes no sense, but you found it really cute and you want to donate to them. Then you realised you're watching the stream on a Piped instance, and you can't do super chat donations without a Google account in use.
There are also times where the instance does not work properly because of either being poorly maintained, or the website has strict rules that restricts any use of public instances.
The other obvious reason is that those services likely violates the terms of service of the original website, which can be extended to legal issues. However, most of those websites don't care if you are consuming contents in such ways, because of the following.
They are all based on proprietary platforms/software, that most likely does not respect users' integrety. As good as such services are, most "content creators" does not care if there are people who don't view the world the way they do. They make contents based on the walled garden the platform provided them, and it does not matter how free the viewer is when viewing such content: most of them are built with a proprietary mindset.
Moreover, such services relies on proprietary platforms, which means that there are no way to ensure the content inside such platforms will remain, and that it's basically telling people to indirectly support proprietary software, as opposed to encouraging them to avoid them, and use software such as Mastiodon, or smol pub.
Too bad with the way the world works, dominates exists, and when they succeeded to be dominating, it's very likely that there are geniunely good works on the platform, that are only avaliable on the platform. Such frontends are good to limit the control of such platforms, while being able to view them without getting all the tracking.
This is probably the best part about free alternative frontends. Yes, it's still a walled garden, but it more of less reminds me of the fediverse, which is an attempt to solve the problems of centralised and decentralised platforms.
The part that gets me the most however, is that I can find different people through looking at the list of instances. There are exceptions but most of them are just individuals/groups hosting the services. Even if you disagree with the idea of proprietary frontends in general, it's an interesting way to see different people who hosts such services with different perspectives/ideologies.
I found that there is a frontend for Wikipedia in Gemini called Gemipedia, and it makes me wonder if people are going to write softwares for websites such as Medium. It's not that I wish it's a thing, but it would be really interesting if someone ever bothers making such services for proprietary platforms on the WWW.