I tend to think that there needs to be a bit clearer identification within the term of "Eastern pantheism." I think that greater clarity here will reveal more in terms of a specific answer. In the most broad of senses, I think that meditation is seen as a way to connect or be one with a larger element that Pantheism suggests is part of the ultimate reality. In meditation, one seeks to be integrated into a larger configuration. For religions that embrace pantheism, this is evident in the idea that there is a larger configuration in the natural setting worthy of worship, an extension of the divine reality. Meditation is thus a way to transcend our world of the mundane and enter that one. For example, if one were to accept the pantheistic elements of Hinduism, one sees that the natural world is an extension of the divine. In meditating, one focuses on this divine extension in daily life. Meditation becomes a way to merge oneself with the extrapolations of the divine, of which pantheistic notions of the natural world is a part. Thus, meditation becomes an essential way in which individuals are able to link themselves, if only for an instant, to something larger than themselves, within which the natural world is representative. Meditation becomes important because it enables the ability to transcend what is into what has already been established through the power and will of the divine.
Meditation is a means to connect to the universe, which is itself a divine entity. Pantheism requires a connection to nature and the universe, and isn't really connected to Polytheistic religions. Pantheism is about the all encompassing, the universe.
Meditation as an exercise helps to bring the meditator to a higher plain of consciousness, that lets them go beyond the material world of the tangible, and really explore the inner self and the awe of connecting to the all encompassing, the infinity of the universe. It does seem so overwhelming to think about consciously, but in a meditative state that is almost a euphoria to be experience, one small particle of something that is infinite.