Alternative to relative clauses

English, unlike other West Germanic languages, has a zero relative pronoun. It is an alternative to that in a restrictive relative clause:

Jack built the house that I was born in.

Jack built the house Ø I was born in.

Examples:

Use with preposition

In formal writing, a preposition in a relative clause often appears together with the relative pronoun. In this case the pronoun is likely to be either whom or which; never that, and since this is now formal usage, it would be unusual to use who.

there is an old house in our street, whose roof Jack fixed.

There is an old house in our street, the roof of which Jack fixed.

Jack is the boy that Jenny fell in love with.

Jack is the boy Jenny fell in love with.

You can use them in informal leanguage as they do not change the meaning of the sentence and it is less formal and shorter
The alternative is used when you do not have to be very specific about who is doing the action because it is obvious.