It can take some months for your appeal to be heard and your circumstances may change in the meantime. The tribunal can only look at your situation as it was up to the time of the decision you are appealing against. It decides whether the decision was correct at the time it was made, based on your circumstances at that time. If your situation has changed between the decision and the tribunal hearing, it cannot take that into account.
SSA, S.12(8)(b)

For disability living allowance renewals, this is the case even if a change takes place after a renewal decision is made but before it takes effect. The only exception is if the change is one that is almost certain to occur, such as a child reaching a particular age (eg 3, when the mobility component becomes payable).
R(DLA)4/05 & CDLA/4331/2002

If you are appealing against an outright refusal of your claim, your circumstances change while your appeal is pending and you think it is now clear