Updated: Nov 7, 2020
Imho, having a handle on some music theory is vital if you wanna make interesting music ("beat makers" can stop reading now, I guess). To be honest though, you don't need to go that far down the theory rabbit hole ...it is an almost bottomless subject, after all. Just get a solid grasp of the basics and build upon that over time. Now, I'm not gonna give recommendations for how to learn the basics - you're on your own there. But for anyone looking to really improve their repertoire of compositional skills, here are three YouTube channels that I've found enormously useful and very approachable for those of us with relatively little 'formal' music training.
One disclaimer here: Two of these channels are kinda pitched at guitar players. However, the material they cover applies equally well to those of us more comfortable around a piano keyboard. Guitars are really just used to illustrate the concepts. You don't even need to be a competent performer in any instrument to benefit from this content either (tbh I probably spend the majority of my writing time inside Ableton's MIDI note window). Likewise, I think this content is applicable to whatever genre of music you work with.
Tommaso Zillio is a seasoned guitar player and professional instructor. His videos cover a lot of interesting topics around chords, scales and modes. He uses an easy to follow whiteboard format, where concepts are carefully explained and illustrated with plenty of concrete examples (demonstrated in the background on guitar). His charming and light-hearted delivery also keeps things very engaging!
2. Signals Music Studio
Jake Lizzio's channel is similar to Tomasso's (above), in that he explains and illustrates the concepts in a clear and engaging manner. But what Jake adds is a composers perspective, ie. how you might actually use each concept when writing music. Jake is also a guitarist, but you don't need to play guitar to keep up.
3. David Bennett Piano
Taking the idea of "theory put into practice" one step further, David Bennett illustrates music theory concepts by analysing their use in well-known songs. He frequently uses musical staff notation to highlight how each song uses a concept, but don't worry if you can't read music (my reading is pretty weak), it's not a prerequisite to follow along.
As someone relatively new to the composer game, I'm continually trying to expand my knowledge and pick up new tricks, while continuing to increase my comfort level with the core concepts. I'd love to know what channels YOU have used to up your game - please share in the comments below!