Why keeping a backlog on index cards is a good idea...

Portability - easy to pick up the cards and take them wherever.

Easy to prioritise - put them on a table and shuffle them.

Focused - there's not much you can write on an index card so only the most important information will be written on them.

Low maintenance - all you need is a pencil and a pile of index cards, and if something changes tear up the current card and write a new one.