A breakpoint is the level where a Pokemon’s fast attack deals 1 more damage than it would before. This level is relative to the Pokemon’s attack IV. Having a higher attack IV often means you’ll reach the breakpoint sooner than you would with a lower attack IV. Because the breakpoint is met earlier with a higher attack IV, many trainers emphasize having a high attack IV as being the true determinant of a Pokemon’s value. However, because the damage plateaus often extend for many levels, Pokemon of any IV % may be nearly equal in DPS as long as they meet the same breakpoint.
For example, using the GamePress breakpoint calculator, we can see that a 15 attack IV Raikou’s Thunder Shock will hit a 5 damage breakpoint at level 31 vs. a Suicune raid boss. A 13 attack IV Raikou must be level 31.5 and a 10 attack IV Raikou must be level 33 to hit this same breakpoint. As a result, you’ll get relatively less value from powering up a Raikou beyond this breakpoint if you intend to use it in Suicune raids. In this situation, putting your Rare Candy and stardust towards another Raikou or saving it for the future will likely serve you better than bringing your Raikou to max.
For players who generally stop powering up past level 30, you will get a decent amount of value from spending the extra stardust to reach these breakpoints if you take the time to calculate them.