When you work with MondoDB you often use indexes to optimize your query.
If you want to create a unique index to prevent duplicated entries for a specific field (like email or username for example) on a new collection you can do it by adding the unique option for the field in your model:
email: { type: String, unique: true}
But if:
You create a new field in an existing collection or
There's already some document in your collection and you want to change the rules of a field
You have to set a default value and use the parse option:
email: { type: String, default: '', unique: true, parse: true}
Otherwise your index will not be setup by MongoDB.
Details
The sparse option does not include all documents of a collection, only those that have the indexed field (even if the value is null) it basically check for non-empty value.
If you need to check more than the field existence I would suggest you take a look at Partial Indexes.