Why do hyperkaehler manifolds have complex structure valued in a 2-sphere? I understand that hyperkaehler manifolds have almost quaternionic structure, whereby there are three complex structures $I,J$ and $K$ which satisfy 
$$
I^2=J^2=K^2=IJK=-1.
$$
It is also said that 
$aI+bJ+cK$ is also a complex structure, so long as 
$$
a^2+b^2+c^2=1,
$$
where $a,b,c\in \mathbb{R}$.
This last requirement implies that the complex structure of a hyperkaehler manifold is valued in a 2-sphere. But what is the reason we have this last requirement on the parameters $a,b$ and $c$?

Note that 
\begin{align*}
& (aI + bJ + cK)(aI + bJ + cK)\\ 
=&\ a^2I^2 + abIJ + acIK + abJI + b^2J^2 + bcJK + acKI + bcKJ + c^2K^2\\
=&\ -a^2\operatorname{id} + abIJ + acIK - abIJ -b^2\operatorname{id} + bcJK -acIK - bcJK - c^2\operatorname{id}\\
=&\ -(a^2 + b^2 + c^2)\operatorname{id}.
\end{align*}
So $aI + bJ + cK$ is an almost complex structure if and only if $a^2 + b^2 + c^2 = 1$.
