The most straightforward reasons would be:

1. Russia would get income from the pipeline, empowering their economy. 2. This sort of infrastructure would represent increased German dependence on Russia for their energy needs. 3. This would also tend to increase economic and diplomatic ties between Germany and Russia.

Destroying the pipeline (even if it's not being used) could theoretically send the message that these infrastructure projects are not safe and that relying on Russia for energy is strategically unwise.