Answer
Burn rate is how fast the budget is spent: 1 uses it up exactly at the end of the SLO window. Page on fast burns (14.4× over 1 hour spends 2% of a 30-day budget) and open a ticket for slow ones (1× over 3 days). Each alert also needs a short window (5 minutes for the 1-hour one) to agree, so it stops firing soon after the problem ends.
Why
A fixed threshold pages for short blips that cost little budget and misses a slow leak that spends it all by the end of the month. Tying alerts to the budget makes them track what users actually lose.