If you sit too much, you don't get enough physical activity. Without physical activity, your muscles and bones weaken, your heart weakens, your circulation and blood pressure get affected, your metabolism activity gets reduced in multiple ways, you burn less energy thus you're more prone to getting fat, you lose flexibility / mobility in your joints... So you get into this whole web of interconnected causes and effects that negatively affect your health.

Physical activity, on the other hand, helps maintain or develop bone and muscle strength, you train your cardiovascular system, you activate processes in your metabolism that help keep your body running efficiently, hormones get produced.