How is spoilage bacteria useful

Mainly medicines and nutrients.

Ive never been asked anything like this but i know there are different types of spoilage bacteria, here are some types

Mesophiles - they prefer moderate temperatures ranging from 41 °F (5 °C) to 50 °F (10 °C). In a favorable medium, these bacteria double their numbers in as little as 30 minutes.

Psychrotrophs - can grow at about 32 °F (0 °C) (the freezing temperature of water), but grow best at moderate temperatures.

Thermophiles - they prefer a warmer temperature. The optimum temperature for growth is usually between 131 °F (55 °C) and 149 °F (65 °F). Some can grow in temperatures as low at 95 °F (35 °F) or as high as 167 °F (75 °C) to 194 °F (90 °C).

Psychrophiles - prefer low temperatures. They can grow at temperatures of 32 °F (0 °C) or lower, but they can also thrive at moderate temperatures of 59 °F (15 °C) to 68 °F (20 °C). They are quite common in environments where the temperature remains consistently low. They are mainly of marine origin and have little consequence in food.

i can do more research on your question though cause im curious myself now