Actually there is a difference:
Cette lettre est à ma fille
This letter belongs to my daughter
It is her property, but we don't know who wrote it. The sentence focuses on the owner, not the writer.
The other form can be ambiguous:
Cette lettre est celle de ma fille
It could mean both:
This letter belongs to my daughter.
This is a letter from my daughter.
However, without further information, the second meaning is the most likely.
EDIT:
Luke's comment adds some precious information:
Cette lettre est celle de ma fille
Can also be seen as an emphasis on this letter regarding some others that could be there. It could then be translated to:
This letter is the one that belongs to my daughter.
This letter is the one written by my daughter.
