It was explained sufficiently well in the game that every time you level up, the spawn evolves in two aspects: physical and skill. It is assumed that the physical aspect can continue to develop over time without leveling up, but skills have a cap on improvement for each level that is unique to each spawn. Therefore, upon reaching level 5, a body user can no longer only change their own body (level 1), but can also change other spawn (level 3) against their will or resistance (presumably level 4). Additionally, they can transform them into something very different from an animal or monster, such as a tree (level 5). It should also be noted that each level increases energy reserves and control over the abilities of previous levels (the latter being debatable but logical).