Safety. What else?

You can see a news report from when it came into force here: https://www.bbc.co.uk/news/uk-scotland-60203081

The guidance is here: https://www.gov.scot/publications/fire-and-smoke-alarms-in-scottish-homes/pages/where-and-what-to-buy/

Which includes this part on batteries:

Replaceable batteries cannot be used because the sensors in the alarm degrade over time and so will not be able to detect heat or smoke. This is why the alarm has a limited lifespan.  Sealed, tamper-proof battery units must be used because they are safer than those which allow the user to change the batteries.

There have been several tragedies over the years where alarms failed because their batteries expired, or people have removed them. Any alarm you buy will have information on how long it lasts, which can be up to 10 years.