Relative clause confusion

Question

I know the general purpose of relative clauses and how to construct them, but lately I've seen them being used in this way,

Der Mann da, der guckt dich immer an

to mean "That man over there is always looking at you".

To me this German sentence translates to "That man there, who is always looking at you" which I understand doesn't make sense on its own, but I'm confused as to why someone would choose to use a relative clause this way as opposed to "Der Mann da guckt dich immer an". Is this a usual way to construct sentences? What is the difference between this relative clause construction and "Der Mann da guckt dich immer an"?

EDIT: As a poster below pointed out, I realize now this was a poor example because this was not a relative clause. There are other examples though, where I have heard actual relative clauses used this way. The main example is the Fettes Brot song "An Tagen Wie Diesen"

Niemand, der mir sagt, wieso Beim Frühstück oder Abendbrot Die Fragen bohren so gnadenlos

which is supposed to mean "No one tells me why these questions persist at breakfast and dinner"

Answer 1

This is not a relative clause. "der guckt dich immer an" is a main clause, as you can see from it not being verb-last, but verb-second. "der" here is not a relative pronoun, but a demonstrative pronoun.

Now you may say, this is not verb-second, it is verb-third! Yes, in a way.
This sentence is an example of so-called "Vor-Vorfeld-Besetzung". In a normal German main clause, the main verb is preceded by exactly one element, and the position this element is in is called the "Vorfeld". However, it is possible to put another element before the Vorfeld-holder (into the Vorvorfeld) if two conditions are met:

• the element in the Vorvorfeld must be coreferential (refer to the same entity) as the element in the Vorfeld
• the element in the Vorfeld must be a demonstrative pronoun

This process, by which the sentence is kind of turned into a verb-third sentence, is also sometimes called left dislocation.

(There are a few other ways of putting something in the Vorvorfeld.)

In contrast, "Der Mann da guckt dich immer an" is a completely normal verb-second main clause, with "Der Mann da" in the Vorfeld.

Answer 2

You are giving indeed two rather different examples, both bearing some difficulty, because they certainly come from spoken context. The part "der Mann da" in the original, first example is an attribute, whereas "der guckt dich an" is the main clause (Hauptsatz – It can stand alone.), not the relative one. For the sake of good style prefer "sehen" over "gucken". ("Der sieht dich an." And even better: "Dieser Mann dort sieht dich an.")

However the (post-edited) second example contains a relative clause beginning with "der". This fact is slightly obfuscated, because of the elliptic "[Da ist] niemand" (or "[Es ist] niemand [da]"). The missing verb in this nominal style clause is not correct in classical german, but contemporary colloquial spoken and written language. (Cf. "Nobody, who can answer my question?")

The main clauses are emphasised by bold letters in the following. The principal rule to identify them is: They can stand alone (without changing the order of words!).

Der Mann da, der sieht dich immer an.

Changing the main clause into a relative clause in another context:

Ich kenne den Mann, der dich immer ansieht.

And the second example

Niemand [ist da], der mir sagt, wieso.

can be used to make a relative sentence in turn:

Niemand, der sagt mir, wieso.

Colloquial [language] allows omitting [words], but [its] grammatical analysis follows [the] same rules.

Answer 3

Regarding the edit, in context, the song by Fettes Brot goes like this:

absolute Wahnsinnsshow
im Fernsehen und im Radio
die Sonne lacht so schadenfroh
an Tagen wie diesen
niemand der mir sagt wieso
beim Frühstück oder Abendbrot
die Fragen bohren so gnadenlos
an Tagen wie diesen

Properly parsed, in prose, it would read like this:

[Da ist eine] Absolute Wahnsinnsshow im Fernsehen und im Radio. Die Sonne lacht so schadenfroh an Tagen wie diesen. [Da ist] niemand der mir sagt, wieso, beim Frühstück oder Abendbrot. Die Fragen bohren so gnadenlos an Tagen wie diesen.

Transl.:

An absolutely insane show in TV and radio.
The sun laughs so schadenfroh
On days like this.
There's nobody to tell me why
During breakfast or lunch.
The questions burn without mercy
On days like this.

There is obviously a heavy caesura between "beim Frühstück oder Abendbrot" and "die Fragen bohren so gnadenlos". There are no relative pronouns across this boundary here. The ellipses may be confusing to non-native speakers. However, "Die Fragen bohren so gnadenlos" is most naturally parsed as a main clause beginning with "Die Fragen", to uphold the verb-second structure; reading it with "beim Frühstück oder Abendbrot", or even more, included is very awkward because it leads to a lot of material before the Vorfeld, when, with some exceptions, the Vorfeld should be the first element.