When looking at Danny Mendick, you can see his name spread out over the depth chart. The same can be said to an even larger extent for Leury Garcia. He is listed right now as a backup for third base, shortstop, and all three outfield positions. He is also listed, for now, as the starting second baseman. This spring is extremely important for him to maintain his status as the starting second baseman on Opening Day.
It is unlikely that we will see him in the minors at any point this season so this is a big spring in terms of him being a starter. If Nick Madrigal plays better than him at any point during Spring Training, he could be in at second and Garcia could be out. What makes Garcia so valuable right now is that he can play basically every position. He is also a decent hitter for a backup position player.
Garcia probably wants to be a starter so he might not be on the White Sox forever. This spring going well for him and earning a starting job at the start of the season could make another team that needs a full-time starter want to make a trade. Garcia wants to look as good as possible for whenever the White Sox decide to make that full time transition to Madrigal. It is good, however, to have a guy like him as a utility backup.