Func_illusionary is a brush entity implemented from Quake and Half-Life.
This entity doesn't generate clipnodes and as such makes the brush non-solid.
As the name might suggest func_illusionary is mostly used to create secrets. In rare cases this entity can be used for optimisation when BSP compiler hits clipnode limit by making complex brush into func_illusionary and overlapping it with a simple clip brush.
Typical to other brush entities, func_illusionary doesn't cut VIS leaves.