They are proposing that the 30,000 going to the military would be engaged with logistics, cyber security, procurement or civil response operations.
For those volunteering, they would work with the NHS, fire service, ambulance, search and rescue, and critical local infrastructure, among others. I am not sure how a mere 25 days per year would result in them making any appreciable difference to those organisations.
Funding for all this is also an issue; it looks like 60% (around £1.5 billion) of it will be "diverted" from the UK Shared Prosperity Fund, which was established to replace the financial support that community organisations lost when we left the EU..