So, person A has the same qualifications as person B, and does the same job, working just as hard, both earning an average salary.
But person A has a disability. So has to spend a chunk of her salary on taxis to work, where person B can walk; adaptations and aids, therapies, that person B doesnt need. So will always be worse off financially than B simply because she is disabled.
Which will have a cumulative impact over her life time. Meaning she is less able to save, to look after her health in other ways, and generally more dependent on more costly services down the line.
The principle behind PIP is to try to make it so those disadvantaged by their disability are not at an even greater disadvantage by being less well off than an able bodied person in the same circumstances. That's why it isn't means tested.
It is also designed to have a preventative effect, the thinking being that investing at an early stage to help people look after themselves and be more independent, reduces costs on the NHS and social care, and in many cases enables people to work.