As larrabee said, the applicant does not need to include a letter, though most people send a cover/contents letter to introduce the application and explain anything complicated about the documents.
The sponsor letter is mandatory and should confirm how you meet each requirement of the visa.
For example:
Sponsor letter:
Paragraph 1: how you are eligible to be a sponsor (your UK citizenship)
Paragraph 2: your employment details/how you are meeting the financial requirement and under which category
Paragraph 3: Where you will live in the UK, including property address and details, and number of occupants in total
Paragraph 4: Short, factual history of the relationship - essential where and when you met, when you entered into a relationship, where and when you married, how often you visit each other and how you keep in contact while apart. Also, why you are settling in the UK together and not the US.
An applicant cover letter can be something like:
To The Entry Clearance Officer,
Please find enclosed my application and supporting documents for a settlement visa to join my husband in Scotland. I would like my visa to start on X date, as per the Intended Travel Date listed on the application form.
[Any explanation of missing documents or anything unusual about the documents goes here - such as large transactions on bank statements]
[Any explanation of previous refused visas/entries, illegal working/overstaying in the UK, or criminal convictions goes here]
I am including the following documents in this package:
[contents list]
Yours Faithfully,
Her Name
