Hi,
Actually, besides planting towers, eating seeds also grants you active abilities as well as permanent stat boosts. If you manage to protect your base long enough, it will level up unlocking new seed types that can change your strategy both when eaten (stronger abilities) and when planted (different tower effects).
You can also check out the Codex page in-game for details about the active powers from seeds and the unique traits of different towers.
As for the map size, we’re planning some balancing updates in future patches. Right now, progressing further can be quite difficult without careful defense and smart seed usage.
Thanks for your feedback!