Shared matches can be helpful when trying to place someone who matches your DNA, into your tree. If you know where the shared match belongs (for example, a maternal cousin) then you know your DNA match must be related to that part of the family.
From your DNA Matches list, click "View Match Details":
On the Match Details page, scroll down to the section labeled Shared DNA Matches. There you can see how each of the shared matches are related to both you and your DNA match. You can also click the name of the shared match to view the Match Details page between that person and your DNA Match. This is especially helpful for comparing the DNA Ancestors for the shared match.