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 focus 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](https://french.stackexchange.com/questions/33037/is-there-any-difference-between-celui-and-%c3%a0-to-mean-possession/33038#comment67805_33038) add 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.*