what role does instrumental music usually play in a spoken word poetry performance and in songwriting

most of the time you can tell by the music what emotions are engaging

The answer is explained below.

Explanation:

In this case, it is necessary to point out that there are, at least, two elements being combined: the instrumental music and poetry/songwriting. In an ideal situations, both parts have to convey the same feelings. Since the music can be understood as a compliment of the poetry and songwriting, it has to reinforce the feelings expressed through the written speech.