Because it reflects on society most how we treat those who are lost at risk and most vulnerable.
If you are working you are already. I'm a better financial position than someone solely reliant on benefits.
In theory to be eligible to claim benefits you should be unable to work or out of work but trying to get back to work. Comparatively, there are very few people solely relying on benefit claims to survive (most of the "pot" for the benefit costs are actually pensions). I assume these are the people we are talking about.
So you have a small number, compared to whole population, who already get a paltry sum to live on, who are facing increases in cost across the board. These are some of the most vulnerable people in our society.
They should absolutely get an increase in line with inflation because without it these people simply won't be able to live. If someone is living with severe disability do you think, as a society, they should also have the worry of whether to heat or eat?
If we make decisions that impact the most vulnerable positively, that becomes the culture more widely and businesses, and other areas of life will see treating people better as something we just do. It's part of the fabric of what we are as a country. Ultimately, we all benefit from a bit of compassion and moral fortitude.