Morning :)
Though In-Game chat can sometimes be toxic (maybe frequent on some occasions), if utilized properly, it can be an essential tool to your success. Yes there are times where you are on a premade team, and you have some sort of program to communicate to each other with (Skype, discord, curse, etc.). However, because it is a PC game and there are times where you are playing with random people, your keyboard becomes a valuable communication tool.
Not everyone is comfortable joining a random person's voice chat. However on console MOBAS, it is more practical to utilize voice chat unless the game has built in commands that direct you to different objectives (SMITE for example - 'attack middle lane' 'defend right lane' etc.).
League's chat is also utilized for more than typing. You can see certain items purchased, objectives (drake/baron) taken, as well as the description of pings among other things.
When you look at the grand scope of the in-game chat for League, it actually does a lot. As some other summoners have mentioned, utilize the mute function or disable All Chat to minimize toxicity.
Hope this helps provide some clarity :)