Fostering a cat is a good idea if you want to help in the cat rescue process without adopting a cat. But you have to have a certain mentality to do it. When you foster a cat you look after a cat for a rescue organisation until he finds a new home.
A fosterer extends the facilities of a cat rescue organisation because he/she takes care of the rescue cat at home.

You have to show the cat to prospective adopters. You have to say goodbye to a cat you may have raised from a very young age. All cat fosterers are going to be tender hearted. You can’t do the job without that quality. As long as the person has the right mindset she’ll be able to give up the cat she has learned to love to an adopter.
On occasions the foster carer adopts the cat she is caring for because it is impossible to say goodbye. When this happens the foster carer is no longer a foster carer but an adopter. It can’t happen more than a few times at the most otherwise the foster carer ends up with too many cats.
I think it is fair to say that good, experienced cat foster carers take joy in the fostering process and in rescuing cats and have no intention of adopting. They probably have a cat or two of their own but have no intention of adopting any more. There has to be a level of professionalism about the work which means putting aside emotions for a while.
The cat rescue organisation will pay for outgoings until the cat is adopted. That in any case is my personal experience. You’ll need to have a suitable home for the job. Experienced foster carers may have built enclosures and such like to care for more than one cat to a high standard.
Is it fair to say that a fostered cat is in a better place than a cat at a rescue center? It could be argued that that a foster home is safer because of the reduced chance of catching a contagious disease from other cats in a large shelter and it should also be less stressful. Shelters can be stressful for cats because of noise, the strange environment and being caged.