For early farming you use your q since it has an increased attack damage but when you get your e you can take down an enemy minion wave quickly and fast and that's a good thing because you don't have your lane pushed down

During the mid game you are already at half tank or almost full tank and you can have your lane pushed down one or two turrets also you can be helping in the team fights where they need tanks most it's a good idea to get some jungle farm but not during team fights

Right now you're full tank and the laning phase is over team fights are breaking out and your team or enemy team is getting baron and drag making final pushes to the nexus you need to be with your team since most others are at full build if they didn't fall behind. So your job as the tank is peeling absorbing damage from turrets and enemy champs