Ajax :

1. New request :
   • Client sends request : Server , Do you have new notification ?
   • Server sends response : Client , NO
   • connection ends
2. New request :
   • Client sends request : Server , Do you have new notification ?
   • Server sends response : Client , NO
   • connection ends
3. New request :
   • Client sends request : Server , Do you have new notification ?
   • Server sends response : Client , NO
   • connection ends
4. New request :
   • Client sends request : Server , Do you have new notification ?
   • Server sends response : Client , Yes here you are
   • connection ends

you see that there is number of useless requests !

web-socket

• connection established :
• client says : Server , Do you have new notification ?

after xx time passed

• Server says : Yes I have

after xx time passed

• Server says : Yes I have
• Client says : Server , send new email if I have

after xx time passed

• Server says : now you got new email

now the it's clear that with web-socket we won't have useless requests