Why was Illinois so important in the civil war?

From the strategic point of view:

1 - Its railway network running from the River Mississippi Eastward, which linked the important Union States' railways along the line of the River Ohio.

2 - Illinois controlled the course of the River Mississippi up to its confluence with the River Ohio about 250 kilometers southward from the vital position of St. Louis, which gave also a strategic predominance over the western Missouri.

3 -The salient from St.Louis to the said confluence was representing a way of further penetration in the Tennessee, once the Union would have secured the positions of Cairo and Paducah.