What is the imitater useful for?

There are a few reasons depending on the level you are playing.
The main reason I personally used them was for pumpkin armor increasing the survivability of your defenses.

But other good options would be:

• Sunflowers/Sun Shrooms at the start of the level to boost production.
• Potato mines for cheap early game defenses
• Marigolds for extra money farming
• Or extra emergency buttons (Jalapeno, Cherry Bomb, Hypno Shroom or squash)

Usually you won't be needing them but they can be useful for getting certain achievements easier. But besides the achievements they are mainly used in endless levels where in each round you focus on a certain type of defensive wall.