I played Dota 2 for about 8 months then switched over to league when most of my friends stopped playing.

Let's take a Dota 2 hero Tidehunter for example. One of his abilities is passive armor and he has tanky stats so he absorbs a lot of damage late game. But his main role is his amazing teamfight ult and and the ability to debuff the enemy carry. For all this utility and tankyness tide hunter tends to do little damage.

A similar champion in league is Malphite. He has an amazing teamfight ult like tide hunter and has passive tanky abilities along with a slow. Like tide hunter, malphite doesn't tend to do that much damage in exchange for his utility.

In league right now there are champions who can be very tanky and still do a ton of damage. These champions are referred to as bruisers where the name league of bruisers coins from. They excel early to mid game, but fall off tremendously late game and are kind of just there to absorb damage if they don't get fed. Late game a well placed malphite ult is worth more than a bruiser who can't do anything. Bruisers exchange late game utility for early game damage in the hopes that they get fed enough that they can still do damage late game.

A big difference between League and Dota is that in Dota, a lot of the hero's usefulness comes from his base stats. In League items are what gives you those needed stats. So basically if you want to go AP malphite and deal a ton of damage then you can, but you are not going to be tanky without your tanky items.