For applications, is this necessary? I am planning to submit my application tomorrow and I don't have any travel booked since I don't know when I may get the visa.
You should NOT book ANY travel until after your visa has been issued... just in case it is delayed or refused and you cannot travel anyway.
What we mean by flight itinerary is an example itinerary of a flight you would like to book if you get the visa.
For example, say you put your Intended Travel Date as December 12th, you might then go onto an airline website and search for flights to the UK on December 12th. Then you just select a flight that looks good and print the itinerary without booking it. You can then include this printed itinerary with your documents.
It's only optional, but you can include one if you like.