I'm doing an online music theory course at present and this tip about the Circle of Fifths jumped out at me.
Basically you can see the diatonic chord family for say C Major highlighted in Red.
So say you have a typical chord progression like this :- C G Am F - C G F C ( I V vi IV I V IV I )
But you need to transpose into E Major instead
Well the Circle of Fifths comes to your rescue as you can see from the highlighted Blue circle.
You now have
E B C#m A - E B A E ( I V vi IV I V IV I )
So in songwriting if you want the family of diatonic chords that play well together then it's a good start or you want to transpose a chord pattern into another key then the circle of fifths has you covered too.
A simple little tip but I never knew about it until now.