Hello Harshit
On the spoke routers it’s not necessary to have a subinterface configured. You can indeed configure the physical interface with Frame-Relay encapsulation and have it function correctly. However, it can be useful to use a subinterface as the number used can help you remember the DLCI you are using. For example, if you’re using a DLCI of 105 on the particular VC on a spoke router, you can configure a subinterface of Serial 0/0.105 so that you can remember that the interface corresponds to DLCI 105. This is strictly a personal preference however, and is not technically necessary.
I hope this has been helpful!
Laz