These spaces may be narrow but that doesn't mean they're completely useless. They will provide access from the front yard to the back (and vice versa) without having to haul things through the house (lawn mowers to snow shovels); they can provide a convenient service entry location for power and gas etc.; they can provide a convenient location for heat pumps and a/c units; and they are typically used to accommodate surface storm water drainage flows.
From a building perspective, these spaces also provide space to vent exhaust from kitchen ranges and household dryers, fireplaces, and to vent fresh air into attic spaces to prevent mould growth due to inadequate circulation patterns from relying on mechanical systems alone.
It's also worth noting that typical UK row housing is 30-35 feet in depth while Canadian narrow lot homes are typically 40-50 feet in depth and can easily reach 50-60 feet on deeper lots.