A letter of wishes is an informal document that sits alongside your Will and sets out some guidance for your Executors and Trustees.
There are a number of reasons why you might want to write a letter of wishes:
If you choose to leave your personal possessions to a specific person or to your executors for them to distribute (rather than detailing the gifts in the Will itself), you should complete a letter of wishes so that they are aware of who should receive what.
If you choose to leave any part of your estate on discretionary trusts (i.e. trusts which give your trustees flexibility in how to distribute the assets), you should create a letter of wishes that the trustees can refer to for guidance after your death.
If you want to leave some guidance to your executors about the sort of charity you would like to leave gifts to (for example if you have named a charity that no longer exists at the date of your death) you can state this in a letter of wishes.
If you want to leave guidance to any Guardians you have appointed for your children, you may want to include this in a letter of wishes.