MORE: Horwill, White left out of Wallabies' World Cup squad
Former captain James Horwill and half-back Nic White were the surprise omissions as Michael Cheika revealed his 31-man selection.
Brumbies man Toomua, 25, said his ability to adapt would have been among the big reasons for his inclusion.
"Versatility seems to be a key thing in the back line," he told Omnisport.
"You look at a lot of guys, there's not many guys who can only play one position so it's probably something that played in my favour in the inside backs so hopefully I can play an important role."
The Wallabies have been drawn alongside England, Wales, Fiji and Uruguay in Pool A for the World Cup, which starts on September 18.
Toomua welcomed the difficult group.
"If you want to win it, you've got to beat good teams don't you? That doesn't change," he said.
"Obviously it's a harder pool, but you can look at it a few ways and say that'll strengthen you for the finals so it's a good opportunity."