Protecting worksheets in Excel allows you to stop anything being changed on a worksheet or specifying that particular cells can be changed whilst others cannot. This is very useful if you want to make sure that certain cells are not changed inadvertently such as formulas and functions. This could be that you want to make sure you do not make a mistake yourself or because you have other people using it and you do not want them to make a mistake or make changes.
It can be useful to protect the information that is being used in a lookup.
It is possible to add a password or you can choose not have a password, if you do use a password make sure you remember it.