While six mana is enough that I would be surprised if this ability were the primary reason people played the card, it can do some pretty powerful things in the late game and opens up the possibility of Honored Knight-Captain seeing play in an equipment deck
Alpharael, Stonechosen is one of these, and you can combine it with cards that make players lose twice the amount of life to straight up win, like Bloodletter of Aclazotz
If they only have a few cards in hand, the choices of what to keep or discard are going to be much more difficult
Its card number is 115/113
Well, only costing a single mana means you can get it down and start swinging before opponents can put up blockers
Read More: The 10 Best Vehicles In Magic: The Gathering 4 Grand Warlord Radha A lot of these creatures are pretty mana-intensive, so it helps to be able to generate a lot of mana with ease