The wrap mode specifies what happens when texture coordinates outside the range 0→1 are used.
Note that if the filter mode is anything but cogl_pipeline_filter_nearest then texels outside the range 0→1 might be used even when the coordinate is exactly 0 or 1 because OpenGL will try to sample neighbouring pixels. For example if you are trying to render the full texture then you may get artifacts around the edges when the pixels from the other side are merged in if the wrap mode is set to repeat.