Single bank account holding all your money is fragile. Technical outages (rare but happen), fraud-related freezes, account disputes, even bank failures can lock you out of access. Backup account at different bank prevents these from becoming crises.
Simple setup
Second current account at different bank. £500-1000 sitting in it. Linked to backup debit card. Set up online in 20 minutes; £0 cost.
When you need it
Most users never use it. Those who do (locked out during foreign travel, fraud freeze on main card during emergency, technical outage during important purchase) are very grateful it exists. Small preparation; potentially trip-saving.