At least one Modifier will always apply when spending Threads. Spending more Threads improves the chances of multiple Modifiers applying.
Breach Modifiers which can be applied by Threads:

Breach Hives in Area lead to a Hive Fortress.

Unstable Breaches in Area contain a Boss.

Area contains an additional Unstable Breach. This Modifier can occur up to three times.

Rare Breach monsters will drop an additional Wombgift when killed.

Area contains an additional Genesis Tree Sapling.

Breach Hive and Fortress encounters will contain additional Breach Coffers.

Rare Breach Monsters will have an additional Breach Modifier.

Ailith's skills in area will be empowered.