Why would you store information about every single event in Earth history in the main computer of a Starship?
I guess you never know when it would come in handy? Obviously it did several times. And yeah, you could always say that such information could be stored elsewhere, but then that assumes that such information could always be accessed.
I think the idea was that computers had such ridiculous amounts of storage space in the future that you could store all that you wanted on there without any problem. Did you know that you could store the compressed text of all the Wikipedia articles in just 9 GB? I have a memory stick I could fit that onto, so wouldn't it at least make sense to just bring that along?