As with all inside surfaces, dirt settles on external surfaces too and they need regular cleaning. On driveways/patios vegetation can grow (moss, weeds, algae, etc.) When algae is wet it can be as slippery as ice so cleaning makes walking on these surfaces safe. But there is more. Power cleaning can revive the colours of the cleaned surfaces, remove vegetation, etc. After the cleaning, surfaces can be sealed and made water repellent https://www.smartseal.co.uk/driveway/block-paving/block-paving-sealer-matt-finish.htm and larger gaps between stones re-pointed, or kiln dried sand swept into smaller gaps. This closes the gaps between paving blocks and stabilizes them. Pointing stops vegetation regrowing in the future.
Mainly it makes external surfaces clean and look nice.
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