There's going to be a lot of people needing help this Winter, not just "the poorest". And targeting support to the people who need it will be next to impossible - so much depends on life circumstances. How many people are in the household? How many hours per day are they in the house? How efficient is the house and the appliances within it? Just look at posters on here. Some are still paying as little as £60 per month, while others are paying £300-£400.
Fixing energy prices needn't be the only support provided. But it's an effective baseline. Sure, it'll reduce the clamour for energy saving measures. But many of the people who will need support this Winter aren't in a position to pay for them anyway (or live in rented accommodation, and are therefore limited in what they can do).