The first reason is undoubtedly about making a reality of post-Brexit and doing more in the Indo-Pacific.
Second, it is about working closely with your allies. The Integrated Review talks about the UK’s special relationship with the United States, but also its close partnership with Australia. Australia, the US and the UK, together with Canada and New Zealand, are also part of an alliance called the Five Eyes. The UK is also likely to be responsible for training the Australian military so they can use the new nuclear subs.