Because I don’t have to remember to pair my phone with the Bluetooth speaker, then queue up whatever. I just tell my Echo to play my daily briefing, and it does the work. I picked a set that is generally done in 15-20 minutes, so it doubles as a good time for my morning routine.
In the described use-case, you don't have to re-pair your phone, you just turn the speaker on and it reconnects automatically. Both phone OSs work that way, reconnecting automatically to known devices.
Also, the daily briefing is a different use-case than was described above, where the GP commenter just wanted to play music.
That works in a single person household. But once an extra person wants to use it then each person has to do the dance of re-pairing every morning - which can be painful if the other person's device was the last one that connected and is nearby.
Being able to have anyone just walk in the shower and say "device play music on Spotify" and have it just work is much simpler and requires much less thinking and effort - which is really nice first thing in the morning.
Also a speaker you can't connect after getting in the shower, but a smart speaker you can.
