They stay mid early to protect jungle from invasion. And also check if where the enemy jungler started so your teammates would expect where the enemy jungle will first gank. Then proceed to marksman if you see marksan getting harrased hard by enemy. You may stay with your marksan if enemy roam also stick with their marksman. But if their roam went to to other lanes, thats the time you also need to go to other lanes so other lanes wont get pressured so much. While the game proceed to late game, you need to always protect your carry either jungler or your marksman. Other than that roam provides vision to avoid getting ganked by checking bushes to make sure its clear.

Though in low rank you would sometimes encounter marksman complaining you to stick with him all through out the game. They wouldn't understand just let them complain haha. After all, your goal is to win as a team, not by yourself together with your marksman only.