SPR is all about generating post flop conditions that are favorable to the texture of your hand. Gut shots and small pairs love deep stacks, so when we're playing hands that typically make sets, or gapped connectors that will make weak draws, we want an overly large SPR to give us the best implied odds possible. When we have big top pair or overpair hands, deep stacks hurt us and we would much rather have the stacks shallow to deny implied odds to our opponents.

This is why an SPR of say 10 is good for marginal drawing hands, and an SPR of 4-7 is better for marginal to strong made hands. The idea is, the stronger your hand the smaller your target SPR should be, because the lower the SPR the easier it is to get all the money in with fewer and/or smaller bets in relation to the pot size. Hope that helps.