When you burn Steem, you make it more rare, and you equally give back to all members. Think of a simple example where 1000 Steem total exist, and 90 users have 10 Steem each (1% of the supply), and 1 user has 100 Steem. If the 1 user with 100 Steem burns his Steem, this means the total supply of Steem drops to 900, meaning the each user now has more Steem of the total amount of Steem (slightly more than 1% of the supply). As Steem becomes rarer, each user owns more of a percent of the full supply.

By contrast, if you've been on here long enough, you would know that giving away Steem to users provides no value to the platform. People tend to sell everything they get, run off, and not return. What this does is create downward pressure on Steem and lowers the price.

On the peg side, I think the purpose of them burning SBD is to maintain a peg window.