While the Beats acquisition made sense for their talent and get a little faster entry into music streaming, but buying Tidal doesn't make much sense at all. Tidal software and technology doesn't offer anything of value. Jay Z presumably would not want to work under Apple long term.

Tidal has licenses with a lot of artists but those wouldn't transfer to Apple Music, they would have to renogitate with them. And the whole reason artists are choosing Tidal is because they dont want Apple Music and want to be with a company all about the artists. If Apple wants to be all about the artists, it doesn't need Tidal for that, it can simply update it's relevant policies.

One could say the want to buy them to get rid of the competition but I don't see Tidal being much competition since while artists may launch with them most like Adele end up on all the services anyway.

Or the most likely reason is this is simply a an unfounded rumor.

Now buying Pandora to replace their Genius feature would be amazing, but Pandora works so well because the amount of music is so small in comparison and Apple right now is anti algorithms publicly and all about human curation, so doesn't look like a pandora acquisition will be coming anytime soon as much as we want it.