The temperatures on the underground section of the Bakerloo Line get uncomfortably high, even during winter. In summer, they can be almost unbearable at time. This is the oldest tube and the ventilation system needs to be updated. Other tube lines are don't seem to have this problem and are much more comfortable temperature wise.
Why is this important?
For the comfort of passengers, who are after all paying the same fares as passengers travelling on other tube lines. There are people with health problems, elderly people, children etc, who are particularly vulnerable.