Question: So I'm just learning this Coldplay piece and it has notes which are not joined but on the same stave. I'm wondering what the purpose of this is or rather what information it's trying to convey? Perhaps the lower notes should be played when the lower lyrics are to be sung, i.e. on the second repeat?

Later on in the sheets it's made slightly more obvious as some of the notes that are separated are slightly smaller.

I'd like to know for sure as I'm turning my sheet music into MIDI files so that I can use Synthesia to practice them.

![enter image description here](https://i.stack.imgur.com/oBGw0.png)

Answer: 
Smaller notes are a different issue, and could be relevant to a second line of lyrics, but you will typically see them only sporadically appear within the top voice.

Anyway, what you're asking about is referred to as *multiple voices* on a staff. Notes in harmony will generally only be grouped together with a stem if the rhythm is the same, and all of the notes are "on the same part".

There are three parts/voices in this excerpt. The bass is on the bass cleff staff, the melody is the top part of the treble clef staff, and the part in the middle is a harmony part. All parts should be played with each line of lyrics, and the top line should be sung.

Generally a staff will contain up to two voices, since they can easily be distinguished from one another with the use of up stems or down stems. But this is certainly not a hard limit; it is quite common to see more than 2 voices in a staff when reading orchestral scores. In ambiguous cases, the music will contain short instructions like "1.", "a 2" to designate which parts should be playing where.
When notes do share the same stem like this, it is because the editor is showing the two notes to be different "voices". In this particular case, they want to distinguish the melody, which is the top part, from the bottom notes that are filling out the harmony. 

Separating the voices is extremely common whenever there is a sung melody that is also being played by the piano. Choral music where all four parts are written on the same staff will often be separated the same way so that the singers can tell which part is theirs.