The 38-year-old Cahill was included in Van Marwijk's first squad named on Wednesday for friendlies against Norway and Colombia later this month, despite having barely played since he moved back to English second-tier club Millwall.
"It's not a guarantee for him that I will nominate him for the world championship, but for me it's also important to give him a signal that he's an important player," Van Marwijk told reporters on Thursday.
"He didn't play a lot and it's important that players have rhythm and play.
"But there always are exceptions. He's a player who was very important for the national team in the qualification and in the playoffs."
Cahill quit A-League club Melbourne City in December and returned to Millwall in an attempt to boost his chances of appearing in a fourth successive World Cup.
The former Everton forward, however, has played for less than a total of 40 minutes since his arrival in south-east London.
"It's a very good signal from him that he went to England to play to make a chance for the world championships," Van Marwijk added.
His lack of playing time in the A-League and at Millwall but inclusion in the squad for the March 23 clash with Norway in Oslo and against fellow World Cup qualifiers Colombia in London four days later was greeted with surprise by local media.
Cahill, however, is one of the more experienced players in Australia's national team setup and Van Marwijk has just three months to shape the team after former coach Ange Postecoglou quit following the Socceroos' victory over Honduras to qualify for the finals.
The team assembles in Oslo from Sunday.
"I try to know them better and tell them something about my plan ... but really train things I want, I think we don't have the time," Van Marwijk said.
"My experience is also that it's not good to tell your whole vision in 30 minutes, I think that is not good for the players, that takes some time.
"It's the situation. I cannot change it."
Australia are drawn in Group C with France, Denmark and Peru for the tournament finals in Russia.
(Reporting by Greg Stutchbury; Editing by Toby Davis)
