Why would one borrow Dai instead of buying from exchange ? if i have $150 worth ether i can borrow $100 worth Dai. With 8% stability fee basically at 8% interest rate. On the other hand i can just exchange ether for Dai and get $150 worth Dai without any interest. what is the logic here
Lets say, you own 1 ETH, which currently has a price of 300$. When you sell this, you can buy DAI worth 300$ on the makret (around 300 DAI). And when you sell the DAI, you will get back around 300$, as expected from a stable coin.
But when you deposit you 1 ETH, to get 200 DAI, you effectively keep your ETH, you just can't use it right now. When the price of ether increases (lets say 10% in 1 year) more than the interest rate (8%), you can get back the 1 ETH, for 216 DAI. So you have to buy 16 additional DAI for 16$. But now you have 1ETH, which is worth 330$. So you have 330$-16$=314$, when you sell it, instead of just 300$. This also works, when you deposit new ETH to get the 16 DAI, instead of buying it on a markt for $.
There may be more reasons, that work when you expect the price of ETH to go down as well, but this the only reason I know right now.