I think it might(?) be better for math and things like that, but dig into social sciences or history or whatever, and often at a surprisingly shallow level you're going to have to go find some real books. eBooks fill some of the gap, but lots aren't there. Some stuff is rare (very niche) and can't even be found in newly-printed books, let alone online—used books and libraries are it. Some special collections—again, without even digging that deep, sometimes—become necessary and would require international travel to pursue (depending on where you live).
It's not hard to poke around a decent used book store or university library and find information that's simply nowhere online.
Google scholar + sci-hub is astoundingly deep.
Plenty of rare old books can’t be found online, but there are also plenty which can if you learn where to look and spend some time hunting.
It is indeed something wondrous for little kids... and adults.
I would say the internet is almost always better at everything non-fiction for adults. But I am talking about 4-8 year olds here mostly, and by extension anyone older that wants some of the library experience.