Why do some glass artists make their mouthpieces hollow? (i.e. RL Funktional)

This all comes down to what the blower wants.  If the glass gets too thick, it can crack if the blower is not used to working with thin and thick glass.  Also, to get shapes sometimes it must be done hallow or it would also get too heavy.

Elev8 does many of their custom mouthpieces hollow