I know this is a dumb question, but I've been using .NET for a year and a half now and still don't feel like I fully understand this. I want to learn it since I'm trying to learn .NET Core 2.1, but I'm just not getting it. I have two Udemy courses that I'm taking (right now I'm working on Complete ASP.NET Core 2.0 with Razor Pages by Bhrugen Patel, next is his ASP.NET Core 2.1 class) but he kind just breezes over what the use and reason of using DI is for.

Any help is greatly appreciated!