Lots of reasons.
Here are some reasons that I mentioned to a student I started tutoring back in 2015.
I think getting a paper copy of Tae Kim’s guide is the best option. Its the same information more or less as the website, but with us both having a print copy it should be a little easier to use than the site.
I decided on Tae Kim after reading through more reviews and finding a poll a blogger took asking language learners about their favourite textbook (attached a picture of that poll). Genki and Tae Kim fared about the same and to me I would rather promote/support the little guy (who makes his resources freely available and has a bias toward real spoken Japanese).