I don't get it, what's the point of promoting posts?

Right now you have received 0.509 SBD and 0.696 SP
Right now 0.509 SBD = 4.392 Steem

Therefore you are receiving (4.392 + 0.696) = 5.088 Steem
You have paid 5 Steem and getting a profit of 0.088 Steem

And the promoter getting 5 steem from you and 1.393 Sp
That means 6.393 Steem

So, that's the overall result!