It’s just politics before the next general election.
There are no planned additional funds in this year’s defence budget, so the promised increases will have to be funded by the incoming Labour government, leaving them the problem of where the funds will come from.
Labour shadow ministers have been claiming for months they will match Sunak’s vow to boost defence spending, but both parties have hedged their promises by adding “as soon resources allow”.
By actually committing the money in spending plans Labour will go into the election campaign having to either commit to funding the programme or dodging the question which the Conservatives will exploit.