What are the use for extended chords in songs? I know the 7th can be used for suspense or a leading tone, but I would like to know when to use extended chords and what their tonality and use is.
Extended chord are just chords that have extra notes added to them to color the harmony. The 3 main ways they are typically used are:
Highlighting notes in the Melody
The harmony and melody should always be working together. The melody will typically play chord tones and what are known as non-harmonic tones. If certain non-harmonic tones stick out and define the melody, you may want to reinforce them. A simple example is if you were playing a CM7 and the melody started on a D then you may want to play a CM9 to reflect where the melody starts.
Extensions can be used to smooth out voice leading with and to aid it in general. For example in the progression Dm9-G13-Cmaj7 each of these chords has an E in them which is very helpful in voice leading.
There are times especially when you are using a dominant chord you can add notes to the chord to give it a little more tension. For example in C you could add a 9th to a G7 to give it a little more tension or even alter it (turn it into a #9 or b9) for even more tension.
There's a lot more to these chords then what I mentioned above, but this should get you started.
Generally the term extended chords refers to chords that have notes added beyond the 7th (such as 9th, 11th and 13th chords). Your question is very general so it's hard to give a concise answer. Often, an extended chord would have a similar function to a triad or a 7th chord built on the same root, and an extended chord would be used to get a richer or more sophisticated sound.
Extended chords are strongly associated with jazz and R&B, so if you were doing an arrangement or writing original music and wanted give it a jazz or R&B sound, the use of extended chords would be a way to do that.