Exercise has some clinical evidence to help slow DM by maintaining muscle mass, and that can still be achieved even in a wheelchair.
A Wheelchair, or Cart, as it is often called, can give a DM dog possibly many extra months. They can continue to walk using all four legs, with the cart just giving extra support when they need it for balance and when they can no longer use their rear legs, stirrups will hold their rear legs off the ground safely so their paws don't get scraped as they drag behind. Eventually when needed, some carts can also provide front leg support in addition to the rear leg support.
Another important aspect a wheelchair helps provide is canine enrichment, by enabling them to discover new sights, sounds and smells, or ones they are familiar with on walks in their favourite places. This contributes to mental stimulation and therefore 'Quality of Life' for a dog.