In English it is very common to use question words in declarative sentences. In fact, it's unavoidable given that we use question words like who, what, where, and which as relative pronouns in relative clauses. Consider the following declarative sentences:
- She was the girl who broke my heart.
- I want the one which is tastiest.
- Paris is the place where I live.
In each of these sentences, the question word starts a relative clause.
These are examples of bound relative clauses, that is, relative clauses which have an antecedent ("She", "the one", and "Paris", respectively).
Your examples of:
- What the eggs are for, is to prepare the cake."
- What my husband is, is a nice guy."
- What a bird is, is a kind of animal."
- Where the station is, is close to the Centre"
contain free relative clauses, that is, relative clauses that don't have an antecedent. In these examples, the relative clauses function as free-standing noun-phrase subjects of the sentences.