Basically, when someone attacks your water shield, you gain 200 mana.
However, if you just spam water shield you will gain 50mana + your mp5.
So if I have 200mp5 while casting, this will bump me to over 350mp5.
Basically, by spamming water shield you give yourself [(5/1.5)*50] extra mp5.
This is good for fights that drain you oom, arenas, faster mana regen, etc.
TLDR: You gain 166mp5 by spamming water shield.