Completing a level lets you choose a random number of rewards. Some are free, some cost a certain amount of gems.
Most rewards for gems are not worth getting, but it is the only way to get the Druid Garden, the lowest level of the Druid Gardens. Egg Nests are usually also cheaper than when bought from the buy menu (but more expensive than when obtained from Dimensional Jars).
The amount of level completions required for each set of three is increased by 3 each time the set is restarted.
Occasionally, a Creature Chest is offered for free as a Level Reward. It appears in the "unlocked" state but when chosen it will arrive in your garden as usual locked chest requiring gems to open. In older Merge Magic! versions there was a glitch causing the chest to arrive in a strange state wherein it did not sit on a tile - it appeared to hover between two tiles. You were not able to access it or move it, and it tended to be annoyingly in the way. Leaving your garden and returning caused the chest to disappear, which was the best outcome overall.
Another problem with level rewards is taking creatures who at the time of level end were carrying something. It might happen that the object in transit is also offered to be independently chosen. If you take both the creature and the ferried item the glitch described above will happen to one of the "copies" brought to your garden.