when you start out a game (early game 0-20min for sure) you want to last hit creeps for cs so you gain better items
the faster and more cs you get the faster you get better items, the fast you get better
items the more fed you will become and will overpower the enemy laner and enemy team, also you want to try to last
hit creeps all game but dont make it a priority over being with your team unless really needed.
the key is to keep gaining as much cs as possible while preventing opposing team from getting cs
this will put you higher in upkeep, and them lower, this leads to winning in team fights
getting better items dont necessarily win team fights, but it is a big aspect of it (positioning, spells usage, and items)
all come into play on winning team fights, but that will be in a different video
items dont only help you win team fights they help with ganks. Better items= more % chance of the enemy
dying when being ganked, (again w/o accounting for positioning and spells usage)
Jungle (ganking) for jungle doing his jungle increases his cs, so he can better gank enemys
putting his team further ahead in cs and uptime.
So winning team fights will put the enemy team at a disadvantage
1 waiting for rez
2 less teamates to defend an objective or team fight
3 loss in cs and experience (early game)
Winning team fights will put team at a advantage
1. do objectives (dragon, baron, red, blue, towers, inhibitors) usually different objectives provides different advantages for the team
2. more upkeep (more gold gained for teamfight)
3. more uptime to do cs, enemy jungle (another objective) gain xp
4. usually if its end game you push towers to end the game quickly (priority objective)
So in conclusion, you get cs early game to get better items, you get better items to gank and team fight better, and you gank and win team fights to push objectives
and win the game.